Why don't microwaves leak out from through the glass? How a microwave oven works Controller Sensors Cooling fans Microwave generator Magnetron construction and operation Magnetron construction - basic textbook description Magnetron construction - modern microwave oven Magnetron construction - common features Cross section diagram of typical magnetron Microwave generator circuit diagram Interlock switches Troubleshooting Guide Instant troubleshooting chart - most common problems and possible causes What can go wrong General system problems Uninvited guests Totally dead oven Totally dead oven after repair Dead controller No response to any buttons on touchpad Oven runs when door is still open Oven starts on its own as soon as door is closed Oven works but totally dead display Whacked out controller or incorrect operation Erratic behavior Problems with internal microwave leakage Some of the keys on the touchpad do not function or perform the wrong action Microwave oven does not respond to START button Microwave generator problems No heat but otherwise normal operation Timer and light work but no heat, cooling fan, or turntable rotation Fuse blows when closing or opening door Loud hum and/or burning smell when attempting to cook Arcing in or above oven chamber More on the waveguide cover and cleaning Fuse blows when initiating cook cycle Fuse blows when microwave shuts off (during or at end of cook cycle) Oven heats on high setting regardless of power setting Oven heats but power seems low or erratic Oven heats but shuts off randomly Oven makes (possibly erratic) buzzing noise when heating Oven light does not work Fans or turntables that do not work What to do if the door handle breaks off Crack or other damage to door window Repairing damage to the oven interior Microwave/convection oven problems Sensor problems Testing and Replacement of Components Testing the oven - the water heating test Testing the main fuse

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Notes on the Troubleshooting and Repair of Microwave Ovens

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Testing and replacing of interlock switches Making measurements inside microwave ovens Testing the high voltage components Testing the high voltage diode Replacing the HV diode HV diode ratings Testing the high voltage capacitor Replacing the high voltage capacitor What if the HV diode or capacitor are leaky? Testing the magnetron Comprehensive list of magnetron failure modes Where to obtain replacement magnetrons Comments on replacement magnetron quality Replacing the magnetron Testing the high voltage transformer Testing the HV transformer using an AC current meter Replacing the high voltage transformer Testing and repairing the wiring and connections Testing thermal protectors and thermal fuses Testing and replacing the triac Testing and replacing the power relay Items of Interest Microwave leakage meters Comments on microwave leakage meters Simple microwave leak detectors How safe is a repaired microwave oven? Efficiency of microwave ovens Microwave oven design and cost reduction Problems with running a microwave oven with metal inside or totally empty More on metal in the microwave Burnt smell from oven - after incident Microwave ovens and grounded dedicated circuits Microwave ovens and GFCIs Can a microwave oven be built into (or hung under) a cabinet? Taking a microwave oven oversees (or vice versa) Microwave oven test-mode High frequency inverter type HV power supplies Dangerous (or useful) parts in a dead microwave oven? The magnets in dead magnetrons Using the control panel from defunct microwave oven as an electronic timer Precise control of microwave oven power Has technology gone too far? Microwave ovens for non-standard applications Short course on Amana Computer system near microwave oven? Why Microwave-Safe Containers Get Destroyed Service Information Advanced troubleshooting Suggested Reference Cost of repair parts Interchangeability of components Can I substitute a slightly different HV capacitor for a blown one? Obtaining replacement parts for microwave ovens

DISCLAIMER
Careless troubleshooting of a microwave oven can result in death or worse. Experienced technicians have met their maker as a result of a momentary lapse of judgement while testing an oven with the cover removed. Microwave ovens are without a doubt, the most deadly type of consumer electronic equipment in wide spread use. The power supplies for even the smallest microwave ovens operate at extremely lethal voltage and current levels. Do not attempt to troubleshoot, repair, or modify such equipment without understanding and following ALL of the relevant safety guidelines for high voltage and/or line connected electrical and electronic systems. We will not be responsible for damage to equipment, your ego, county wide power outages, spontaneously generated mini (or larger) black holes, planetary disruptions, or personal injury or worse that may result from the use of this material.

Back to Microwave Oven Repair FAQ Table of Contents.

Introduction
Radar Range anyone?
Remember when you actually had to use the real oven to defrost a TV dinner? Think back - way back - before VCRs, before PCs (and yes, before Apple computers as well), almost before dinosaurs, it would seem. There was a time when the term 'nuke' was not used for anything other than bombs and power reactors.

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and at performance levels indistinguishable from when it was first taken out of the box.htm
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.or . some of its wording appears remarkably familiar! Microtech also offers instructional videos and books on microwave oven and VCR repair.
http://www. Microwave ovens are extremely reliable devices.Notes on the Troubleshooting and Repair of Microwave Ovens
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For a long time. You will be able to make an informed decision as to whether a new oven is the better alternative. With your new-found knowledge.org/sam/micfaq.is that symptoms can sometimes be deceiving and a solution that works in one instance may not apply to your specific problem. It will enable you to quickly determine the likely cause and estimate the cost of parts. However. Connoisseurs of fine dining will turn up their collective noses at the thought of using a microwave oven for much beyond boiling water . My only reservation with respect to tech tips databases in general . With minor exceptions.repairfaq.if that. Rather.in terms of microwave emissions and molecular damage to the food. Unlike other consumer electronics where a new model is introduced every 20 minutes . you can greatly simplify your troubleshooting or at least confirm a diagnosis before ordering parts. the most common problems will be addressed and enough basic principles of operation will be provided to enable you to narrow the problem down and likely determine a course of action for repair.electronics. specific manufacturers and models will not be covered as there are so many variations that such a treatment would require a huge and very detailed text.repair. it is difficult to deny the convenience and cooking speed that is provided by this relatively simple appliance. There is a good chance that your oven will operate for 10 years or more without requiring repairs of any kind .this has nothing to do with Microtech in particular . Should you still not be able to find a solution. Jim Bryant's Microwave Ovens page is another site worth visiting. you will be able to do what is required for a fraction of the cost that would be charged by a repair center . Touchpads are now nearly universal because they are cheaper to manufacture than mechanical timers (and also more convenient). It is quite possible your problem is already covered at the Microtech site. you will have learned a great deal and be able to ask appropriate questions and supply relevant information if you decide to post to sci.the microwave oven has not changed substantially in the last 20 years. Not entirely coincidentally. an understanding of the hows and whys of the equipment along with some good old fashioned testing is highly desirable to minimize the risk of replacing parts that turn out not to be bad. The comprehensive Safety Info is a must read as well. you will have the upper hand and will not easily be snowed by a dishonest or incompetent technician. In many cases. Therefore. the microwave oven has taken its place in virtually every kitchen on the planet. you will have the satisfaction of knowing you did as much as you could before taking it in for professional repair. This document provides maintenance and repair information applicable to most of the microwave ovens in existence. Whether these issues have been resolved or just brushed aside is not totally clear. You will be able to decide if it is worth the cost of a repair as well. In any case. Cooking is cooking. there was controversy as to whether microwave ovens were safe .some even have useful improvements . an old microwave oven will heat foods just as well as a brand new one. I assume. Nonetheless.be able to revive something that would otherwise have gone into the dumpster or continued in its present occupation as a door stop or foot rest.
On-line microwave oven repair database
Microtech maintains a web site with a large amount of information on microwave oven repair including an on-line Tech Tips Database with hundreds of solutions to common problem for many models of microwave ovens. In that case. he will answer questions via email and includes links to many USA microwave oven manufacturers and parts suppliers. There are also an extensive list of microwave oven related links to other interesting sites (including this document!). While he deals mostly with models in the UK. However.

Erratic touchpad operation due to spill . this may be an effective way of obtaining a solution quickly as long as you follow the extremely important safety information provided by MIDES (or this document). Blown fuse due to power surge or old age: Replace fuse. Many problems can be solved quickly and inexpensively. Bugs in the works . On rare occasions. probably on the controller.htm
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. Arcing in oven chamber: clean oven chamber and waveguide thoroughly.
Repair or replace?
With small to medium size microwave ovens going for $60-100 it hardly makes sense to spend $60 to have one repaired.repairfaq. may have shorted due to a power surge blowing the controller fuse. Remove remains of MOV. Replace carbonized or damaged waveguide cover. For the novice. Touch up the interior paint. if you can do the repair yourself.Notes on the Troubleshooting and Repair of Microwave Ovens
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Expert system for microwave oven fault diagnosis
The MIDES (Microwave Oven Diagnosis Expert System) site represents an interesting and possibly useful approach for isolating the cause of many common failures..
The simplest problems
Bad interlocks switches or door misalignment causing fuses to blow or no operation when the start button is pressed. the main fuse may even be intermittent causing very strange symptoms. understood.. replace MOV for future surge protection. Fixing an old microwave for the dorm room may just make sense after all. Locate and replace defective switches and/or realign door. replace fuse and test. However. the equation changes dramatically as your parts costs will be 1/2 to 1/4 of what a professional will charge and of course your time is free.. The educational aspects may also be appealing. Thus. More detailed explanations are provided elsewhere in this document. Smooth rough metal edges. You will not be forced to acknowledge that you have read. It will take you through a customized step-by-step procedure based on your symptoms (and specific microwave oven model in some cases) and the results of its suggested tests. and followed their safety precautions and warnings before performing each test. Even full size microwave ovens with full featured touchpanel can be had for under $200.. You will learn a lot in the process.
Back to Microwave Oven Repair FAQ Table of Contents. replacement should be considered seriously before sinking a large investment into an older oven.let touchpad dry out for a week.org/sam/micfaq.
Installation and Preventive Maintenance
Microwave oven installation and use
http://www. An MOV.the controller circuit board is a nice warm safe cozy place to raise a family.

arcs. home centers. Built up food deposits can eventually carbonize resulting in sparks. there may be requirements or suggestions that are specific to your model and will enable you to get the most performance from your new microwave. It will be cheaper to buy.easy access and not too high or too low.e. A grounded outlet is essential for safety. Leave at least 2 inches on all sides and top if possible.do not push it up against the wall or wedge it under a tight fitting wall cabinet (or inside one for that matter!).
Microwave oven maintenance
Most people do not do anything to maintain a microwave oven. While not much is needed. Select a convenient location . A GFCI is not needed as long as the outlet is properly grounded and may result in nuisance tripping with some microwave ovens. It will also minimize the possibility of Radio Frequency Interference (RFI) between it and any electronic equipment which might be on the same circuit. Make sure the outlet is in good condition in either case.repairfaq. regular cleaning at least will avoid potentially expensive repairs in the future: Clean the interior of the oven chamber after use with a damp cloth and some detergent if necessary. Even something as simple as microwave popcorn can explode and/or catch fire if heated for too long . to confirm that the outlet is properly wired and grounded.g. In any case.as well as potentially more serious damage to the magnetron. Romex. heating.. Inexpensice outlet testers are available at hardware stores.htm
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. and damage to the mica waveguide cover and interior paint . follow these recommendations: Read your users manual from cover to cover especially if this is your first microwave. Do not let children use the microwave oven unless properly supervised. If there is any chance of food deposits having made their way above the waveguide cover in the roof of the chamber. Microwave ovens are high power devices and a separate circuit will eliminate nuisance fuse blowing or circuit breaker tripping when multiple appliances are being used at the same time.Notes on the Troubleshooting and Repair of Microwave Ovens
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To assure safety and convenient. cheaper and easier to service.org/sam/micfaq. remove the waveguide cover and
http://www. This is particularly important if the door of the oven opens down instead of to the left side (only a few models are built this way. Temporary use of a 3 to 2 prong adapter is acceptable only if the outlet box is properly grounded to begin with (BX. Put the microwave oven on its own dedicated 3 wire grounded circuit. and possibly more reliable since ventilation and adjacent heat producing appliances will not be as much of a factor. you may discover that your oven has features you were not aware were even possible. spread the metal strips of each of the prongs apart if possible and/or replace the outlet. It is very easy to cause a fire through the use of excessive times or power settings. Allow adequate ventilation . What a concept! If nothing else. however). 5 minutes instead of my precisely determined 3:41 on high :-). If there is. and electrical parts distributors. or conduit with ground) AND the adapter's ground wire or terminal is securely attached to the outlet box ground screw. Check that the plug (or adapter) fits tightly and that there is no appreciable heating of the outlet during use of the microwave oven. Select a stand-alone unit rather than a built-in if possible.

Keep your kitchen clean. I know it is difficult to get at but I warned you about that!). Such an open mesh should not affect the cooling of the electronic components significantly. grinding. Clean the exterior of the cabinet and touchpad in a similar manner. However. Periodically check for built up dust and dirt around the ventilation holes or grills. 2. Be especially careful around the area of the touchpad since liquid can seep underneath resulting in unresponsive or stuck buttons or erratic operation. Heat. Inspect the cord and plug for physical damage and to make sure the plug is secure and tight in the outlet . If you do this by accident. clean up whatever is attracting the unwanted tenants (and anything they may have left behind including their eggs!!).
http://www. or other noises .particularly if the unit is installed inside a cabinet (yes.Notes on the Troubleshooting and Repair of Microwave Ovens
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thoroughly clean inside the waveguide as well. This is because: 1. Do not use strong solvents (though a bit of isopropyl alcohol is fine if needed to remove sticky residue from unwanted labels. I know. DO NOT use a spray where any can find its way inside through the door latch or ventilation holes. the outlet (and possibly the plug as well) should be replaced.it is either being produced or is non-existent. Listen for any unusual sounds coming from inside the oven. immediately unplug the oven and let it dry out for a day or two. or a dripping wet cloth. Yes. There is little energy storage in the microwave generator compared to the amount being used.htm
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. WARNING: See the section: SAFETY before going inside. the mesh will likely clog up more quickly than the original louvers so make sure it is cleaned regularly. for example). Keeping the ventilation free will minimize the chance of overheating. If possible. There is no scientific basis for such a recommendation. There is no such thing as residual microwave radiation from a microwave oven . While these appliances are not exactly quiet. If it is too late and you have a recurring problem of cockroaches getting inside the electronics bay. squealing. especially from a combination microwave/convection oven or from other heat producing appliances can damage the plug and/or cord. CAUTION: Do not spray anything into the holes where the door latch is inserted or anywhere around the touchpad as this can result in internal short circuits and costly damage . this isn't exactly microwave specific but cockroaches and other uninvited guests might just like to take up residence inside the electronics bay of the oven on the nice warm controller circuit board or its neighborhood and they aren't generally the tidiest folks in the world.org/sam/micfaq. Once the beep has sounded (or the door has opened). scraping. tell them to get lost and then put window screen over the vents (or wherever they are entering). it is safe.may indicate the need for some more extensive maintenance like belt replacement or motor lubrication. Attending to these minor problems now may prevent major repairs in the future.especially if they were not there when the oven was new . Clean them up and use a vacuum cleaner to suck up loose dust.
How long does microwave energy hang around?
You have probably been warned by your mother: "Wait a few seconds (or minutes) after the beep for all the microwaves to disappear". for that matter.or anywhere else inside.repairfaq. If there is evidence of overheating at the outlet itself.

Microwave Oven Troubleshooting
SAFETY
The following applies to microwave oven troubleshooting . unplug the oven from the AC outlet before removing the cover and do not plug it in to operate it with the cover off if at all possible.org/sam/micfaq. When you remove the metal cover of the microwave oven you expose yourself to dangerous . Always ensure that it is totally discharged before even thinking about touching or probing anything in the high voltage power circuits. remove the connections to the magnetron (see below) to prevent the inadvertent generation of microwaves except when this is absolutely needed during troubleshooting. replace the oven or have it professionally repaired. In this case.htm
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.that can retain a dangerous charge for a long time.
http://www.has a capacity of less than 15 W-s (Watt-seconds) even for the largest ovens. To prevent the possibility of extremely dangerous electric shock. There is a high voltage capacitor in the microwave generator. there is a very real risk of potentially lethal electrical shock even after several minutes or more of being unplugged! See the section: SAFETY if you will be troubleshooting a microwave oven. Power consumption is typically 800 to 1500 W depending on oven size. it is more like . the magnetron may not be drawing any current from the HV power supply and the HV capacitor can remain charged for a long time. for a 1500 W oven with a capacitor storing 15 W-s. See the troubleshooting sections later in this document. Please see Typical Microwave Oven Electronics Bay for parts identification.once the cabinet cover is removed. You may also be exposed to potentially harmful levels of microwave emissions if you run the oven with the cover off and there is damage or misalignment to the waveguide to the oven chamber. Discharge the high voltage capacitor (with the oven unplugged) and then use clip leads to make any connections before you plug it in and apply power. Very high voltages (up to 5000 V) at potentially very high currents (AMPs) are present when operating .long before the beep has ended or the door has cleared the front panel. These dangers do not go away even when unplugged as there is an energy storage device . If you must probe live. If you have the slightest doubts about your knowledge and abilities to deal with these hazards.01 seconds!) WARNING: This only applies to a *working* microwave oven! If there is no heat.repairfaq.1 second .deadly combination.
Back to Microwave Oven Repair FAQ Table of Contents.potentially lethal . below.Notes on the Troubleshooting and Repair of Microwave Ovens
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The typical high voltage capacitor .the only component that can store energy . There is also safety information on proper use of the oven in subsequent sections.a high voltage capacitor . WARNING! WARNING! WARNING! WARNING! WARNING! WARNING! WARNING! WARNING! Microwave ovens are probably the most dangerous of consumer appliances to service. Then after removing power and unplugging the oven discharge the HV capacitor once again.electrical connections. Careless troubleshooting of a microwave oven can not only can fry you from high voltages at relatively high currents but can microwave irradiate you as well. the capacitor will be fully drained in much less than . above. (Based on the numbers. Therefore.

The microwave oven circuitry is especially hazardous because the return for the high voltage is the chassis . short across its terminals with the blade of a well insulated screwdriver. and other consumer and industrial equipment can be both rewarding and economical. microwave ovens. Hold them in place with string or electrical tape. If you need to probe. particularly through your heart. Set up your work area away from possible grounds that you may accidentally contact. Any involuntary muscle contractions caused by a shock. Just be sure that it is also safe! Don't work alone . use a 25K to 100K 25 W resistor with a secure clip lead to the chassis. while perhaps harmless in themselves. I highly recommend avoiding any probing of the HV circuits .25 AMP at 50/60 Hz . may cause collateral damage . Therefore.in the event of an emergency another person's presence may be essential. Safety guidelines These guidelines are to protect you from potentially deadly electrical shock hazards as well as the equipment from accidental damage. Know your equipment: TVs and monitors may use parts of the metal chassis as ground return yet the chassis may be electrically live with respect to the earth ground of the AC line. Don't wear any jewelry or other articles that could accidentally contact circuitry and conduct current. For the microwave oven in particular. discharge (across) large power supply filter capacitors with a 25 W or greater resistor of 5 to 50 ohms/V approximate value. solder.there are many sharp edges inside this type of equipment as well as other electrically live parts you may contact accidentally. Then. Note that the danger to you is not only in your body providing a conducting path.repairfaq.the continuous power rating of the HV transformer may exceed 1500 W with short term availability of much greater power. Repair of TVs. the HV may exceed 5000 V peak with a continuous current rating of over .nearly everything can be determined by inspection and component tests with the oven unplugged. In addition.org/sam/micfaq. monitors. put insulating material between the boards and anything they may short to. Wear rubber bottom shoes or sneakers. or otherwise touch circuits with power off. Always observe high voltage protocol. to be doubly sure that the capacitor if fully discharged. I also recommend leaving a clip lead shorting across the capacitor
http://www. Prop them up with insulation sticks .it is not isolated. The purpose of this set of guidelines is not to frighten you but rather to make you aware of the appropriate precautions. or get caught in moving parts. Touch each of the capacitor terminals to the non-grounded end of the resistor for several seconds. do not assume that the chassis is a suitable ground for your test equipment! If circuit boards need to be removed from their mountings. In addition.Notes on the Troubleshooting and Repair of Microwave Ovens
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WARNING: Experienced technicians have been electrocuted deader than a brick from even careful probing of the HV circuits of a powered microwave oven. Microwave ovens use the chassis as ground return for the high voltage.htm
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. Always keep one hand in your pocket when anywhere around a powered line-connected or high voltage system.plastic or wood. Mount the resistor on the end of a well insulated stick.

an isolation transformer can and should be used to test the primary side circuitry if necessary including interlocks. it won't be terribly useful as noted above. Use an isolation transformer if there is any chance of contacting line connected circuits. for example. However.will not be operating at full capacity. However. or the high voltage side of a microwave oven. etc. or brush up against. Connect/disconnect any test leads with the equipment unpowered and unplugged. of course. lean on.org/sam/micfaq. Isolation transformers and microwave ovens There's little point to using an isolation transformer with a microwave for testing the high voltage circuitry. never assume anything without checking it out for yourself! Don't take shortcuts! As noted. Finally.deductive reasoning . your equipment. At most. connect the
http://www. Perform as many tests as possible with power off and the equipment unplugged. but your primary diagnostic tool . Not only will you be more careless. triac/relay. however. Disconnect the HV transformer to eliminate the possibility of high voltage shock and to reduce the load.repairfaq.) The use of a GFCI (Ground Fault Circuit Interrupter) protected outlet is a good idea but will not protect you from shock from many points in a line connected TV or monitor. the semiconductors in the power supply section of a TV or monitor can be tested for short circuits with an ohmmeter. Failures are usually easily found by performing test with the oven unplugged. the best policy is to NEVER EVER attempt to measure anything in the HV section while the oven is powered . prevent your scope probe ground from smoking should you accidentally connect an earth grounded scope to a live chassis. Use clip leads or solder temporary wires to reach cramped locations or difficult to access locations.htm
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. A circuit breaker is too slow and insensitive to provide any protection for you or in many cases. put electrical tape over all but the last 1/16" of the test probes to avoid the possibility of an accidental short which could cause damage to various components. Don't attempt repair work when you are tired. A Variac(tm) is not an isolation transformer! (See the next section with regards to isolation transformers and microwave ovens. It would have to be HUGE due to the high power nature of a microwave oven and since the high voltage return is the chassis which is grounded.it's almost never needed in any case. a GFCI (Ground Fault Circuit Interrupter) will NOT protect you from the high voltage since the secondary of the HV transformer is providing this current and any current drawn off of the secondary to ground will not be detected by the GFCI. motors. none of these devices will protect fools from themselves! Take extreme care whenever working with the cover off of a microwave oven.Notes on the Troubleshooting and Repair of Microwave Ovens
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terminals while working as added insurance. An isolation transformer is even limited value as well since the chassis IS the HV return and is a large very tempting place to touch. Actually. If you insist on making live measurements. And. Clip the reference end of the meter or scope to the appropriate ground return so that you need to only probe with one hand. A GFCI may. For example. use of a GFCI is desirable to minimize the risk of a shock from the line portions of the circuitry if you don't have an isolation transformer. If you must probe live. you will blow a fuse if you should forget to remove it when powering up the microwave.

wire cutters and wire strippers. sleep on it. start with some analytical thinking. Stanley or Craftsman are fine. The best location will also be relatively dust free and allow you to suspend your troubleshooting to eat or sleep or think without having to pile everything into a cardboard box for storage.not on a deep pile shag rug. See the document: Troubleshooting and Repair of Consumer Electronics Equipment for additional info on soldering and rework techniques and other general information. A medium power soldering iron and rosin core solder (never never use acid core solder or the stuff for sweating copper pipes on electronic equipment) will be needed if you should need to disconnect any soldered wires (on purpose or by accident) or replace soldered components. More notes is better than less. most of the power components in microwave ovens use solderless connectors (lugs) and replacements usually come with these as well. Old dead microwaves can often be valuable source of hardware and sometimes even components like interlock switches and magnetrons as these components are often interchangeable. Don't work when you are really tired . If you get stuck. These do not need to be really expensive but poor quality tools are worse than useless and can cause damage. A crimping tool will be needed as well but the $4 variety is fine for occasional use. there may be a defective door interlock switch or just a tired fuse. Qualified service people have been electrocuted using proper test equipment on microwave ovens!
Troubleshooting tips
Many problems have simple solutions.they may not all be identical. Sometimes. Needed tools include a selection of Philips and straight blade screwdrivers. Many problems associated with consumer electronic equipment do not require a schematic (though one may be
http://www. An assortment of solderless connectors (lugs and wirenuts) is handy when repairing the internal wiring.
Test equipment
Don't start with the electronic test equipment. For a microwave oven. just letting the problem bounce around in your head will lead to a different more successful approach or solution. film canisters. While not advocating being a pack rat. make notes of which screw went where .it is both dangerous (particularly with microwave ovens) and mostly non-productive (or possibly destructive .repairfaq. this does have its advantages at times. Select a work area which is well lighted and where dropped parts can be located . and plastic ice cube trays come in handy for sorting and storing screws and other small parts after disassembly. needlenose pliers. Don't immediately assume that your problem is some combination of esoteric complex convoluted failures. Something like a large plastic tray with a slight lip may come in handy as it prevents small parts from rolling off of the work table. Pill bottles.Notes on the Troubleshooting and Repair of Microwave Ovens
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meter before power is applied and disconnect or move its probes only after power is removed AND the HV cap has been discharged (even if the meter catches fire or explodes!).org/sam/micfaq. If you need to remove the cover or other disassembly.htm
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. A basic set of high quality hand tools will be all you need to work on a microwave oven.very destructive). However.

In any case.Notes on the Troubleshooting and Repair of Microwave Ovens
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useful). These are not super accurate or sensitive but are better than nothing. and only then change the connections. A thermometer (glass not metal) to monitor water temperature during power tests.extremely unlikely.for your safety and to prevent damage to the device under test as well as your test equipment . The technique I recommend is to use a high wattage resistor of about 5 to 50 ohms/V of the working voltage of the capacitor. however) and testing of interlock switches. wiring. Note: always have a load inside the oven when testing . Low voltage. fuses. For the high voltage capacitor in a microwave oven. several minutes may be required for the voltage to drop to negligible levels. WARNING: the high voltage in a microwave oven is NEGATIVE (-) with respect to the chassis. While these are supposed to include internal bleeder resistors. use a 25 W or larger 100 K ohm resistor for your discharge widget with a clip lead to the chassis. don't just interchange the probes = it may be last thing you ever do. You will wonder how you ever lived without one! Cost: $25-50. However. A microwave power detector.
Safe discharging of the high voltage capacitor
It is essential . Even better is to use a proper high voltage resistor rated for at least 5 kV. The reason to use a large (high wattage) resistor is again not so much power dissipation as voltage holdoff.repairfaq. High voltage probe (professional. Should you accidentally use the wrong test probe polarity with your meter. These can be purchased or you can make one from a small neon (NE2) or incandescent bulb with its lead wires twisted together.a cup of water is adequate. There are special magnetron and microwave test instruments but unless you are in the business. resistance. You do not need an oscilloscope for microwave oven repair unless you end up trying to fix the logic in the controller . This does not need to be expensive but since you will be depending on its readings. Sometimes these homemade solutions do not survive for long but will definitely confirm that microwave power is present inside the oven chamber.org/sam/micfaq. these can fail. Inexpensive types are readily available at home centers or by mail order. and most of the components of the microwave generator. discharge the HV capacitor. Unplug the oven. Other useful pieces of 'test equipment': A microwave leakage detector. these are unnecessary extravagances. Also see the sections: "Microwave leakage meters" and "Simple microwave leak detectors".htm
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. reliability is important.
http://www. You don't want the HV zapping across the terminals of the resistor. A DMM or VOM is necessary for checking of power supply voltages (NOT the high voltage. This will prevent the arc-welding associated with screwdriver discharge but will have a short enough time constant so that the capacitor will drop to a low voltage in at most a few seconds (dependent of course on the RC time constant and its original voltage). this is only rarely actually required. A series string of 10 to 20 1/2 W normal resistors in series can also be used. not homemade!). Even a relatively inexpensive DMM from Radio Shack will be fine for most repair work. The majority of microwave oven problems are easily solved with at most a multimeter (DMM or VOM). or continuity checks will identify most problems.that the large high voltage capacitor in the microwave generator be fully discharged before touching anything or making measurements.

This discharge tool will keep you safely clear of the danger area. confirm with a WELL INSULATED screwdriver across the capacitor terminals.1 second. Don't just wrap it around . Solder the other end of the resistor to a well insulated contact point such as a 2 inch length of bare #14 copper wire mounted on the end of a 2 foot piece of PVC or Plexiglas rod which will act as an extension handle.repairfaq. Secure the resistor to the insulating rod with some plastic electrical tape. The capacitor discharge indicator circuit described in the document: Capacitor Testing. If there is a big spark. it is a good idea to put a clip lead across the capacitor terminals just to be sure it stays fully discharged while you are working in the area. Again. Use the discharge probe on each side of the capacitor in turn for a second or two. Unplug the unit! Usually. Safe Discharging and Other Related Information can be built into the discharge tool if desired. A suitable discharge tool can be made as follows: Solder one end of the appropriate size resistor (100K ohms. you may blow everything . always double check with a reliable high voltage meter or by shorting with an insulated screwdriver! Reasons to use a resistor and not a screwdriver to discharge capacitors: 1. it is unlikely that this would be detected. You need to decide.this connection must be secure for safety reasons. Finally. this should drain the charge quickly and safely.including yourself. Then. 25W in this case) to a well insulated clip lead about 2 to 3 feet long. A microwave still under warranty should probably be returned for warranty service for any covered problems except those with the most obvious and easy solutions. It will reduce your spouse's stress level in not having to hear those scary snaps and crackles. At worst. There are usually no warranty seals on a microwave so unless you cause visible damage or mangle the screws or plastic. you will blow the fuse upon powering up if you forget to remove it. If your discharging did not work. Since the time constant RC is about . 2. It will not destroy screwdrivers and capacitor terminals. your original attempt was less than entirely successful.at least in principle. capacitors have been known to spontaneously regain some charge. WARNING: DO NOT use a DMM for checking voltage on the capacitor unless you have a proper high voltage probe. 3.
Getting inside a microwave oven
You will void the warranty . you will know that somehow. Yes. There is a very slight chance the capacitor could be damaged by the uncontrolled discharge but at least there will be no danger. It will not damage the capacitor (due to the current pulse). the sheet metal cover over the top and sides is easily removed after
http://www.Notes on the Troubleshooting and Repair of Microwave Ovens
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Clip the ground wire to an unpainted spot on the chassis.htm
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.org/sam/micfaq.

Dalbani. Reassemble in reverse order. A schematic showing all of the power generation components is usually glued to the inside of the cover.repairfaq. So. :) Details will vary depending on manufacturer and model but most of the major components will look fairly similar to those depicted in the photo. Kind of like Russian Roulette." Therefore. They are not usually all the same! At least one of these includes a lockwasher to securely ground the cover to the case. Fortunately. "The repair will be $195 because you blew out the touch panel by removing the cover. Confirm that the screws you removed go back in the proper locations.
http://www. they can make any sort of claim they want as to what might have been damaged even if all you did was remove and replace the cover without touching anything inside. if it's then taken to a service center. there's a high probability that the oven no longer works at all.htm
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. And. Fortunately. particularly the one that grounds the cover to the chassis. Note that for this model. there may also be one screw that is slightly longer than the others to engage a safety case interlock switch and prevent the oven from getting power if it is not present or one of the shorter screws is used in its place. The cover will then lift up and off. the oven lamp is actually inside the electronics bay right next to the high voltage on the magnetron filament . But when the cover is replaced with the screws in random locations. If less than entirely honest. everything may be really squeezed together. This is critical to avoid microwave emissions should the waveguide or magnetron become physically damaged in any way. Note how fingers on the cover interlock with the main cabinet . it is essential to make note of any differences in screw types so they can be put back in the same place. the inside of a microwave is wide open and this is not difficult.
Principles of Operation
Instant (2 minutes on HIGH) microwave oven theory
Please see Typical Microwave Oven Electronics Bay for parts identification. How much of the controller is included varies but is usually minimal. they will know someone has been inside.org/sam/micfaq.these are critical to ensure prevention of microwave leakage after reassembly. Please see Typical Microwave Oven Electronics Bay for parts identification. and Premium Parts. nothing is powered inside (which is a good thing for safety!). with the cover removed.light bulb changing here is really best left to a professional if you would otherwise not go inside! Discharge the high voltage capacitor as described in the section: Safe discharging of the high voltage capacitor before even thinking about touching anything. Most of these are on the back but a few may screw into the sides. Take particular care to avoid pinching any wires when reinstalling the cover. all the parts in a microwave can be easily replaced and most of the parts for the microwave generator are readily available from places like MCM Electronics. Make sure ALL of the metal fingers around the front edge engage properly with the front panel lip.Notes on the Troubleshooting and Repair of Microwave Ovens
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unscrewing 8-16 philips head or hex head sheet metal screws. Note that on some ovens (I've heard that some Sharp models do this).
Back to Microwave Oven Repair FAQ Table of Contents. Not all ovens are this wide open. If yours is a compact unit.

the microwave energy does penetrate these few cm rather than being totally applied to the exterior of the food. There is little transfer of energy directly to these materials. Microwave safe metal shelves will have nicely rounded corners. or even paper containers will heat only through conduction from the hot food. From: Barry L.45 GHz was probably chosen for a number of other reasons including not interfering with existing EM spectrum assignments and convenience in implementation. Water has numerous resonances over the entire spectra range. Thus plastic. and so forth. heating is via conduction from the burner or coil and there will be ample opportunity for small bubbles to form on the bottom long before the entire volume has reached the boiling point. the wavelength (about 5 inches) results in reasonable penetration of the microwave energy into the food. For references. glass. A microwave oven should never be operated without anything inside as the microwave generator then has no load . This can take place in a microwave since the heating is relatively uniform throughout the liquid. One very real effect that may occur with liquids is superheating. Such a superheated liquid may boil suddenly and violently upon removal from the oven with dangerous consequences.all the energy bounces around inside an a great deal is reflected back to the source. but the lowest frequency resonance is the rotational resonance is around 24 GHz.
http://www. the time to heat food is roughly proportional to its weight. Water molecules are not resonant at this frequency. However. nearly all the energy generated by the oven is available to heat the food and heating speed is thus only dependent on the available power and how much food is being cooked. Since the pie can only cool from the outside.htm
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. Ignoring losses through convection.half the power is absorbed in the outer 1 inch of depth.repairfaq.) "Industrial ovens still often operate at 915 MHz and other frequencies near 6 GHz are also used. The misconception may arise when sampling something like a pie filling just out of the microwave (or conventional oven for that matter). check books on microwave spectroscopy by Townes and Gordy. The 3 dB (half power) point is about 1 inch for liquid water . another 1/4 of the power in the next inch. What is significant about 2. Most metal objects should be excluded from a microwave oven as any sharp edges (areas of high electric field gradient) may create sparking or arcing which at the very least is a fire hazard. Heating is not (as popularly assumed) from the inside out. unlike a conventional oven. This also means that the food does not need to be a conductor of electricity (try heating a cup of distilled water) and that electromagnetic induction (used elsewhere for high frequency non-contact heating) is not involved. the interior filling will appear to be much hotter than the crust and will remain that way for a long time. It is possible to heat a pure liquid like water to above its boiling point if there are no centers for bubbles to form such as dust specks or container imperfections. Other resonances occur in the millimeter wave range through the infrared. This heating is caused mainly by the vibration of the water molecules.45 GHz to heat the food.45 GHz? Not that much. A wide range of frequencies will work to heat water efficiently.net)." Since the oven chamber cavity is a good reflector of microwaves. With a stovetop.Notes on the Troubleshooting and Repair of Microwave Ovens
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A typical microwave oven uses between 500 and 1000 W of microwave energy at 2.org/sam/micfaq. 2. Thus two cups of water will take around twice as long to bring to a boil as one. In addition. The penetration depth of the microwave energy is a few cm so that the outside is cooked faster than the inside. Ornitz (ornitz@tricon.

) Greetings. A schematic diagram of the microwave generating circuitry and portions of the controller is usually glued to the inside of the cover. steps it up to a high voltage. and followed by a sheet of glass or plastic to make sure that food splatters and vapor condensation are easy to clean . The microwave generator takes AC line power.org). from the outside. The reason this works is that the wave that the dish picks up is longer than the hole in the mesh. runs the display. wire mesh (usually a sheet of metal which is either stamped or drilled with a hole pattern . and applies this to a special type of vacuum tube called a magnetron . (From: Filip (I'll buy a vowel) Gieszczykiewicz (filipg@repairfaq. Power level is determined by the ratio of on time to off time in a 10-30 second cycle.org/sam/micfaq.like a color TV CRT mask!).to minimize any stray and harmonic waves from escaping. It consists of two parts: the controller and the microwave generator. i would greatly appreciate it.5 cm). it's not really "glass" but rather a 'sandwich' of glass. like bouncing tennis and golf and ping-pong balls and marbles off the mesh . The controller runs the digital clock and cook timer. Since the holes in the mesh are much much smaller than the wavelength of the 2. i have not been able to get a better answer than 'a wire mesh'.
Controller
The controller usually includes a microcomputer. sets microwave power levels. and in high performance ovens. BTW." There *is* a wire mesh embedded in the glass panel.45 GHz microwaves (about 5 inches or 12. HIGH will be continuous on.. Did you ever see a "mesh" satellite disk up close? You will note that it looks much like it's made out of simple wire mesh that you can get in a hardware store (in the USA.yet still be able to see through it) they will not let anything out of the oven. MEDIUM
http://www. For example. monitors the moisture or temperature sensors.without affecting the "watching the food" ..it WOULD work because the ball is bigger than the holes. Power level in most microwave ovens is set by pulse width control of the microwave generator usually with a cycle that lasts 10-30 seconds. The controller is what times the cooking by turning the microwave energy on and off.repairfaq.imagine scraping the mesh!
How a microwave oven works
The operation of a microwave oven is really very simple. is probably more expensive to manufacture!).you want to catch all the possible sizes ...
Why don't microwaves leak out from through the glass?
"I am trying to find out what the glass on a microwave consists of exactly. as long as the holes are smaller than that (actually. you want them as small as possible . The wave in the microwave is about 2. though very inexpensive units may simply have a mechanical timer (which ironically.Notes on the Troubleshooting and Repair of Microwave Ovens
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This may cause expensive damage to the magnetron and other components. if you can help. it is essentially opaque to microwaves and essentially all the energy is reflected back into the oven cavity.5cm "long" . it's called "chicken fence" :-). Consider bouncing a tennis ball on the "wire mesh" in the microwave .htm
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.little changed from its invention during World War II (for Radar).

cooling is extremely important. the air.htm
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.
http://www. or is being operated from very high AC line voltage increasing power to the oven. rectifier. The power ratios are not quite linear as there is a 1 to 3 second warmup period after microwave power is switched on. and so on. Typically has a secondary of around 2. a microwave oven is really only more efficient than conventional means such as a stovetop or gas or electric oven for heating small quantities of anything. One interesting note: Since 30 to 50 percent of the power goes out the vents in the back as heat. failures of the sensor probes themselves are common. wasted energy goes into heating the pot or oven. It consists of 5 parts: high voltage transformer. If there are any belts. The HV return will be fastened directly to the transformer frame and thus the chassis. waveguide to oven chamber. Most common are probes for temperature and moisture. a pair of leads for the Magnetron filament. even to the point of a continuous range of power. An oven that shuts off after a few minutes of operation could have a cooling problem.25 amp more or less depending on the power rating of the oven. rectifier diode. The controller activates the microwave generating circuitry using either a relay or triac. However. The operating voltages for the controller usually are derived from a stepdown transformer. fast enough to have the same effect as continuous control for all practical purposes. there have been some back in the 1970s that did this with a 1 second or so pulse width modulated cycle. system described below. a bad magnetron. some models use finer control. Therefore. However. a defective overtemperature thermostat. capacitor. High Voltage Transformer. and LOW may be 5 seconds on. this is relatively independent of the quantity of food and may be considered to be a fixed overhead. capacitor.000 VRMS at 0. A couple of drops of electric motor oil or 3-in-One will go a long way. You cannot miss this as it is the largest and heaviest component visible once the cover is removed. inspect for deterioration and replace if necessary.Notes on the Troubleshooting and Repair of Microwave Ovens
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may be 10 seconds on.org/sam/micfaq.
Microwave generator
This is the subsystem that converts AC line power into microwave energy.repairfaq.
Cooling fans
Since 30 to 50 percent of the power into a microwave oven is dissipated as heat in the Magnetron. These are typically "inverter" models which use a more sophisticated type of power supply than the simple high voltage transformer. 15 seconds off. There will be a pair of quick-connect terminals for the AC input. 10 seconds off. Always inspect the cooling fan/motor for dust and dirt and lubricate if necessary. A convection oven will include a temperature sensor above the oven chamber. However. there is a crossover point beyond which it is more efficient to use conventional heat than high tech microwaves. With a normal oven or stovetop. Since these sensors are exposed to the food or its vapors.3 V at 10 A is typical). There will also be a low voltage winding for the Magnetron filament (3. magnetron. and a single connection for the HV output.
Sensors
More sophisticated ovens may include various sensors.

Dr. Magnetron .thus about 1 turn per volt input and output (this is about 1/4th as many turns as in a "normal" power transformer.5 inch long with wire leads. a researcher at the Raytheon Corporation. and output antenna.basic textbook description
http://www. (Some really old microwave ovens may use the classic design as well.htm
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. it's not enough to provide any reduction in the likelihood of electrocution should you come in contact with the HV winding! Rectifier . It is considered by many to be the invention most critical to the Allied victory in Europe. Note that this use of 'working voltage' may be deceiving as the actual voltage on the capacitor may exceed this value during operation. it is a box bolted to the chassis.usually rated 12. Sometimes. However. see the articles at the Microtech Web Site. Always discharge the capacitor as described below before touching anything inside once the cover is removed. possibly to compensate for various magnetron load conditions. Percy Spencer.) This is followed by my autopsy of a dead magnetron of the type that is probably in the microwave oven in your kitchen.no disassembly required!) So there would about 3 turns for the magnetron filament and 2080 turns for the high voltage winding for the transformer mentioned above. about . Magnetron construction .000 to 15.5 amp. The first is what you will likely find if you go to a library and read about radar. (It's usually possible to count the primary turns by examining how it is wound .45 GHz. This provides some current limiting.65 to 1. The capacitor is metal cased with quick-connect terminals on top (one end). it is cylindrical in shape but this is less common. Topics include basic microwave theory as well as a complete discussion of microwave oven magnetron construction and principles of operation.. and the antenna (hidden by the waveguide) on top. The frequency of the microwaves is usually 2. Most commonly. There is also generally a "magnetic shunt" in the core of the transformer. (Items (1) to (6) in the following sections apply to each type while items (7) to (9) apply to both types.org/sam/micfaq. In the typical 'I have to know why this happened' mentality of a true scientist. Here are two descriptions of magnetron construction.
Magnetron construction and operation
The cavity magnetron was invented by the British before World War II.Notes on the Troubleshooting and Repair of Microwave Ovens
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These transformers are designed with as little copper as possible.) For more detailed information with some nice diagrams.repairfaq. The Amana Radarange and the entire future microwave oven industry were the result.000 VAC. Capacitor .the microwave producing tube includes a heated filament cathode. was standing near one of the high power radar units and noticed that a candy bar in his shirt pocket had softened. The story goes that shortly after the War. The reason they can get away with so few turns is that it operates fully loaded about 90 percent of the time but is still on the hairy edge of core saturation. Sometimes.000 PRV at around . The magnetron is most often box shaped with cooling fins in its midsection. One end will be electrically connected to the chassis. this will be rectangular or cylindrical. multiple resonant cavities with a pair of permanent ceramic ring magnets to force the electron beams into helical orbits. The primary for 115 VAC is typically only 120 turns of thick wire . the filament/HV connections on the bottom section.2 uF at a working voltage of around 2. he decided to investigate further.

This is usually supplied by a permanent magnet though electromagnets have been also used. Cooling of the anode block must be provided by forced air. 5. Note: this coating is the only material contained in the microwave oven magnetron that might be at all hazardous. formed in a helix with about 8 to 12 turns. a toxic metal. The wavelength of the microwave energy is approximately 7.5 mm) diameter. Multiple cylindrical resonator cavities at a fixed radius from the cathode bored in the anode block.modern microwave oven This description is specifically for the 2M214 (which I disassembled) or similar types used in the majority of medium-to-high power units. The cathode is supplied with a pulsating negative voltage with a peak value of up to 5. The entire assembly is placed in a powerful magnetic field (several thousand Gauss compared to the Earth's magnetic field of about . The original designs used huge somewhat horseshoe shaped permanent magnets which were among the most powerful of the day. 2. This is supplied with pulsed or continuous power of many thousands of volts (negative with respect to the anode. Also see this photo of the Typical Magnetron Anode and Resonant Structure.a dozen or so turns of heavy wire on a ferrite core .7 mm). nearly all other magnetrons used in modern domestic microwave ovens should be very similar.to prevent microwave leakage back into the filament circuit and electronics bay of the oven. 4.000 V. The item numbers are referenced to the diagram in the section: Cross section diagram of typical magnetron.4 cm) used in a microwave oven this would result in a cavity diameter of approximately .020" (.repairfaq.Notes on the Troubleshooting and Repair of Microwave Ovens
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This is the description you will find in any textbook on radar or microwave engineering.org/sam/micfaq. 5/32" (4 mm) diameter and just over 3/8" (9. 3.5 Gauss). Magnetron construction .62" (15. Typical filament power is 3. water. may be used in large radar magnetrons but should not be present in the types found in domestic microwave ovens. The cathode is coated with a material which is good for electron emission. The filament and cathode are one in the same and made of solid tungsten wire. Beryllium. (For the frequency of 2. An antenna pickup in one of the cylindrical cavities which couples the microwave energy to the waveguide. See the text below for parts names and dimensions. 6. A cylindrical anode block surrounding but separate and well insulated from the cathode. However. 1. This is a view looking up through the anode cylinder from the filament end of the tube. or oil since the microwave generation process is only about 60 to 75 percent efficient and these are often high power tubes (many kilowatts).94 times the diameter of the cavities.3 VAC at 10 A. about .5 mm) in length. A centrally located cylindrical electron emitting cathode. Channels link the cavities to the central area in which the cathode is located.htm
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.45 GHz (12. The original Amana Radarange and other early microwave ovens likely used this design as well. The filament gets its power via a pair of high current RF chokes .
http://www. 1.

062" (1. Rather than cylindrical cavities (as you would find in most descriptions of radar magnetrons). The entire assembly is placed in a powerful magnetic field (several thousand Gauss compared to the Earth's magnetic field of about . Of course. Copper shorting rings at both ends near the center join alternating vanes. The magnetic field forces them to travel in curved paths in bunches like the spokes of a wheel. This structure results in multiple resonant cavities which behave like sets of very high quality low loss L-C tuned circuits with a sharp peak at 2. 7. A connection is made near the middle of a single vane to act as the output power takeoff. Magnetron construction . 5. and attaches to the pressed-on bu?ll-nose antenna cap. electrons stream from the cathode to the anode.5 mm) thick copper with an inside diameter of 13/8" (35 mm) and a length of about 1" (25.common features The following items apply to all types of magnetrons. all the even numbered vanes are shorted to each other and all the odd numbered vanes are shorted to each other.4 mm). Surrounding this space are the . The inductance and capacitance are provided by the precise configuration and spacing of the copper vanes.5 mm) thick and approximately 1/2" (12. exits the tube via a cylindrical ceramic insulator sealed to the top cover.062" (1. The simplest way to describe what happens is that the electron bunches brush against the openings of the resonating cavities in the anode and excite microwave production in a way analogous to what happens
http://www. 4.5 Gauss). A set of thin aluminum fins act as a heat sink for removing the significant amount of wasted heat produced by the microwave generation process since it is only about 60 to 75 percent efficient. 8. Thus. these are about 2-1/8" (54 mm) OD. When powered. and anode cylinder. 1/2" (12. 3. and the resonant cavities. The anode and magnetron case are at ground potential and connected to the chassis. The anode is a cylinder made from . individual inductors and capacitors are not used.htm
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. all the rings are also all shorted at the outside where they are joined to the inner wall of the cylinder. There will always be a cooling fan to blow air through this assembly.5 mm) wide.repairfaq.5 mm) thick edges of the 10 vanes with gaps of approximately . This is provided by a pair of ceramic ring magnets placed against the top and bottom covers of the anode cylinder. 1-13/16" (46 mm) ID. The gap between the cathode and anode.7 mm) long by 3/8" (9. The filament leads/supports enter through a cylindrical ceramic insulator sealed to the bottom cover and then pass through a hole in the bottom end plate.Notes on the Troubleshooting and Repair of Microwave Ovens
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2. For the 2M214.062" (1.7 mm) thick.45 GHz. These are press fit on the magnetron anode and also in contact with the magnetron case.04" (1 mm) between them.org/sam/micfaq. 6. there are a set of 10 copper vanes . Steel plates (which probably help to shape the magnetic field. It passes through a hole in the top end plate. shorting rings. are all in a vacuum. These are brazed or silver soldered to the inside wall of the cylinder facing inward leaving a 5/16" (8 mm) central area clear for the filament/cathode. see below) and thin steel covers (to which the filament and antenna insulators are sealed) are welded to the ends of the cylinder. At this high frequency.

being across the HV diode. reduces the peak value of this somewhat . so the peaks will approach the peak-peak value of the transformer or nearly 5000 V in the example above.25 A | HV |'--> Micro||( typical | Diode | waves (Controller not shown) || +------------+---------+ _|_ . low cost. At the end of normal operation. 10 A. often bolted to the magnetron case. Note that there is a difference in the labels on the filament connections of the magnetron. The negative peaks will get squashed somewhat under load. )||( 2. some of this will likely be discharged immediately but will not likely go below about 2. typical | TP Relay or || +------------+------+FA F| Magnetron _ Fuse I __ Triac || | +-|----|-+ o------.repairfaq. See the section: High frequency inverter type HV power supplies.htm
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.Notes on the Troubleshooting and Repair of Microwave Ovens
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|| +------------------------+ ||( 3.000 V due to the load since the magnetron does not conduct at low voltages. This is a half wave voltage doubler.up to V(peak) will still be present across the capacitor when power is removed. while F goes to the lone Filament terminal on the HV transformer.the magnetron is across the diode.which will be approximately V(peak) = V(RMS) * 1.including high frequency inverters but it is hard to beat the simplicity.3 VAC. Then. Functionally. there will likely be an over-temperature thermostat .somewhere in the primary circuit. not shown. What this means is that the peak voltage across the magnetron is the transformer secondary + the voltage across the capacitor. the typical schematic (as above) shows FA going to the node attached to the Anode of the HV diode. The voltage across the HV rectifier will then be: V(peak) + V where V is the waveform out of the transformer.000 VAC _\_/_ +----|:--+ o------------+-------------------+ ||( .minutes. it becomes a simple half wave rectifier/filter so far as the voltage acrtoss the capacitor is concerned .----/ ----+ || +------||----+ | |_ _| | | )||( HV Cap | | \/ | AC I \ I=Interlock )||( __|__ | ___ | Line | TP=Thermal Prot. and reliability of the half wave doubler configuration.414 where V(RMS) is the output of the high voltage transformer.up to 5000 V at AMPs available! WARNING: Never attempt to view this waveform on an oscilloscope unless you have a commercial high voltage probe and know how to use it safely! The easiest way to analyze the half wave doubler operation is with the magnetron (temporarily) removed from the circuit. In addition. The magnetron load.ALWAYS DO THIS IF YOU NEED TO TOUCH ANYTHING IN THE MICROWAVE GENERATOR AFTER THE OVEN HAS BEEN POWERED.like an open filament . Other types of power supplies have been used in a few models .Chassis ground
Note the unusual circuit configuration . Take extreme care . The peaks are negative with respect to the chassis. The output waveform looks like a sinusoid with a p-p voltage equal to the p-p voltage of the transformer secondary with its positive peaks at chassis ground (no load). The bleeder may be defective and open as this does not effect operation of oven and/or the time constant may be long . not the capacitor as in a 'normal' power supply. However.org/sam/micfaq. HOWEVER: DO NOT ASSUME THAT THIS IS SUFFICIENT TO DISCHARGE THE CAPACITOR .where most of its conduction takes place._---+---/ -. WARNING: What this implies is that if the magnetron is not present or is not drawing power for some reason . There is also usually a bleeder resistor as part of the capacitor. There may also be a thermal fuse or other
http://www.thermal protector . it probably doesn't matter which way they are connected. Some ovens may not have a bleeder at all.

org/sam/micfaq. This makes it more difficult for an ignorant consumer to just bypass the door interlocks should they fail or to run the oven with an open door as a room heater . etc. Complete meltdowns are not unusual! If any defective door switches are found. but the switch contacts will likely be damaged by the high initial current! This also means it probably wouldn't be a bad idea to replace the interlock switch which might have been affected if your oven fails with a blown fuse due to a door problem. should that switch ever actually be used.
Interlock switches
Various door interlock switches prevent inadvertent generation of microwaves unless the door is closed completely. they will sequence correctly. oven light. cooling fan.repairfaq.perhaps as high as 75 percent. Other parts of the switched primary circuit include the oven interlock switches. Primary Interlock: In series with the high voltage (magnetron) power supply so cuts power when the door is open.Notes on the Troubleshooting and Repair of Microwave Ovens
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protector physically elsewhere but in series with the primary to the high voltage transformer. And. Note that if the Door Sensing switch should malfunction. a short will be put across the power line causing the fuse to blow forcing the oven to be serviced. opening the door to interrupt a cook cycle results in arcing at the contacts. Interestingly. (That interlock may be known as a "dummy switch" for obvious reasons and is often not even mentioned in the schematic/parts manifest. The interlocks must be activated in the correct sequence when the door is closed or opened. not only will the fuse blow. turntable motor (if any). The typical door switches and their function: Door Sensing: Input to the microcontroller to indicate the state of the door. peculiar behavior may occur (like the fan or turntable operating at the wrong time) but should never result in microwaves being generated with the door open.) Of course. another interlock is set up to directly short the power line if it is activated in an incorrect sequence. Failed door interlocks account for the majority of microwave oven problems .
Back to Microwave Oven Repair FAQ Table of Contents.and protects the manufacturer from lawsuits.most common problems and possible causes
http://www. Otherwise. This is not surprising considering that two of the three switches carry the full oven current any deterioration of the contacts results in increased resistance leading to their heating and further deterioration. At least one of these will be directly in series with the transformer primary so that a short in the relay or triac cannot accidentally turn on the microwaves with the door open. it is probably a good idea to replace all of them as long as the oven is already apart. Interlock Monitor: Shorts out the AC line (and blows the main fuse) should the Primary Interlock not open due to incorrect sequencing of the door switches or a failed switch.htm
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. The interlocks are designed so that if the door is correctly aligned.
Troubleshooting Guide
Instant troubleshooting chart .

Notes on the Troubleshooting and Repair of Microwave Ovens

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The following chart lists a variety of common problems and nearly all possible causes. Diagnostic procedures will then be needed to determine which actually apply. The 'possible causes' are listed in *approximate* order of likelihood. Most of these problems are covered in more detail elsewhere in this document. While this chart lists many problems, it is does not cover everything that can go wrong. However, it can be a starting point for guiding your thinking in the proper direction. Even if not listed here, your particular problem may still be dealt with elsewhere in this document. Problem: Totally dead oven. Possible causes: 1. 2. 3. 4. 5. No power to outlet (blown fuse or tripped breaker or GFCI). Blown main fuse - likely due to other problems. Open thermal protector or thermal fuse. Defective controller or its power supply. Clock needs to be set before other functions will operate (some models).

Problem: Totally dead oven after repair. Possible causes: 1. Cabinet screws replaced in incorrect location (safety interlock not engaged). 2. Any number of screwups. :) Problem: No response to any buttons on touchpad. Possible causes: 1. 2. 3. 4. 5. 6. 7. Door is not closed (some models). You waited to long (open and close door to wake it up). Controller is confused (pull plug for a minute or two to reset). Defective interlock switches. Faulty controller or its power supply. Touchpad or controller board contaminated by overenthusiastic cleaning. Defective/damaged touchpad.

Problem: Oven runs when door is still open. Possible causes: 1. Damaged interlock assembly. 2. Cooling fans (only) running due to bad sensor or still warm. Problem: Oven starts on its own as soon as door is closed. Possible causes: 1. 2. 3. 4. 5. Defective triac or relay. Controller is confused (pull plug for a minute or two to reset). Defective controller or its power supply. Touchpad or controller board contaminated by overenthusiastic cleaning. Defective/damaged touchpad.

Problem: Oven works but display is blank. Possible causes: 1. Defective controller or its power supply.

Problem: Some keys on the touchpad do not function or perform the wrong action. Possible causes: 1. 2. 3. 4. Touchpad or controller board contaminated by overenthusiastic cleaning. Defective/damaged touchpad. Controller is confused (pull plug for a minute or two to reset). Faulty controller.

What can go wrong
The most common problems occur in the microwave generating portion of the system, though the controller can be blown by a lightning strike or other power surge. Bad interlock switches probably account for the majority of microwave oven problems. Also, since the touchpad is exposed, there is a chance that it can get wet or damaged. If wet, a week or so of non-use may cure keys that don't work. If damaged, it will probably need to be replaced - this is straightforward if the part can be obtained, usually direct from the manufacturer. Unfortunately, it is an expensive part ($20-50 typical). The interlock switches, being electromechanical can fail to complete the primary circuit on an oven

http://www.repairfaq.org/sam/micfaq.htm

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Note: when working on controller related problems. unplug the connection to the microwave generator (HV transformer primary) from the power relay or triac .
Totally dead oven
First. No adjustments should ever be required for a microwave oven and there are no screws to turn so don't look for any!
General system problems
The following problems are likely power or controller related and not in the microwave generator unless due to a blown fuse or bad/intermittent connections: Totally dead oven. First. See the section: Testing and replacing of interlock switches. Failed interlocks are considered to be the most common problems with microwave ovens.it is often a separate connector. these will have be be jumpered with fine wire and then soldered. Sometimes. Oven works but display is blank. Microwave oven does not respond to START button.repairfaq. there is likely a problem with the controller circuitry or its power and you will have to get inside the oven.Notes on the Troubleshooting and Repair of Microwave Ovens
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which appears to operate normally with no blown fuses but no heat as well. Dry completely.perhaps a power surge . If there are any actual breaks. and from their point of view makes an ideal habitat. If this does not help. Erratic behavior. Clean the circuit board and connectors thoroughly with water and then isopropyl alcohol. Creatures with six or more legs (well. such visitors are quite possible. perhaps as high as 75% of all failures. or friend. the microcontroller will get into a whacko mode for some unknown reason . If you got the microwave oven from a flea market.htm
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. Inspect the circuit traces for corrosion or other damage.
Uninvited guests
Some cockroaches (or other lower life forms) may have taken up residence on the controller circuit board. Faulty interlocks or a misaligned door may result in the fuse blowing as described above due to the incorrect sequencing of the door interlock switches. It is warm. safe. garage sale. unplug the microwave oven for a couple of minutes. Oven starts on its own as soon as door is closed. some two legged varieties as well) are not known for their skills in the areas of housekeeping and personal hygiene. No response to any buttons on touchpad Oven runs when door is still open. Some keys on the touchpad do not function or perform the wrong action. The fuse or circuit breaker at
http://www. a relative.org/sam/micfaq. or if your kitchen isn't the cleanest in the world.and simply needs to be reset. no electronic components were affected though there is always a slight possibility of other problems. Hopefully. the curb. cozy. This will prevent any possible accidental generation of microwave energy as well as eliminating the high voltage (but not the AC line) shock hazard during servicing. The problem may never reoccur. Whacked out controller or incorrect operation. check power to the outlet using a lamp or radio you know works.

then putting in a new fuse will enable the touchpad/display to function but may blow again as soon as a cook cycle is initiated if there is an actual fault in the microwave circuits.microwave ovens are high current appliances and should be on a dedicated circuit if possible. then any short circuit in the microwave generator will also disable the controller and display. With some ovens the screen will be totally blank following a power outage . you have a problem with the outlet or other wiring on the same branch circuit. See the section: Microwave generator problems. there is at least one cabinet screw that is slightly longer than all the others.remove and inspect the light bulb and socket.and it appears to operate normally . there is a short circuit in the power cord or elsewhere. If the same thing happens again. Some models have a thermal fuse as well and this may have failed for no reason or a cooling fan may not be working and the oven overheated (in which case it probably would have died while you were cooking something for an important guest . power relay.org/sam/micfaq. try a new fuse. A refrigerator should never be plugged into the same circuit for this reason as well you really don't want it to be without power because of your popcorn! If you find the fuse blown or circuit breaker tripped. If this blows immediately. Next. unplug everything from the circuit to which the microwave is connected (keep in mind that other outlets may be fed from the same circuit). even a shorted oven lamp . If plugging in the microwave causes the fuse to blow or circuit breaker to trip immediately. Other possible causes: bad controller power supply or bad controller chip. try to set the clock. a fuse has probably blown although a dead controller is a possibility. This is common. If the oven now works. This engages a safety interlock which prevents the oven from receiving power if the correct screw is missing or in the wrong hole. Furthermore.Notes on the Troubleshooting and Repair of Microwave Ovens
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your service panel may have blown/tripped due to an overload or fault in the microwave oven or some other appliance.there may be nothing wrong with it. Assuming these are not your problems. some ovens will not allow you perform any cooking related actions until the clock is set to a valid time. Check the length of all the screws and locate the interlock
http://www. The microwave oven may be powered from a GFCI outlet or downstream of one and the GFCI may have tripped. If you attempt to run a heating appliance like a toaster or fryer at the same time.then one of the components in the microwave generator is defective (shorted). If this is the case. you *will* blow the fuse or trip the circuit breaker. You may just have too many appliances plugged into this circuit . or a defective triac (if your oven uses a triac). there may be a short very near the line cord. confirm that the controller is operational by unplugging the microwave generator.assuming you would use a microwave oven for such a thing!). The tripped GFCI could be in the garage or almost anywhere else! Pushing the RESET button may be all that's needed.
Totally dead oven after repair
On some microwave ovens.htm
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.repairfaq. If the main fuse is upstream of the controller. (Removing a broken oven lamp has been known to happen. initiate a cook cycle (with a cup of water inside). If the fuse still blows immediately. Therefore. in the controller. Or. If it does not blow. If a new fuse does not now blow when a cook cycle is initiated . Replace the fuse or reset the circuit breaker. and/or triac from the controller. the fuse may simply have been tired of living.) The GFCI outlet may not be in an obvious location but first check the countertop outlets.

and there may even be spares to use for just this situation! Assuming that the main fuse and power transformer primary checks out. diagnosing this will be tough without a schematic (and possibly much more). Some of these thin traces may be there specifically to act as fuses .even setting the clock . There is a good chance that the surge didn't propagate beyond the transformer and thus the rest of the controlled should be unaffected. You waited too long .
http://www. As always.Notes on the Troubleshooting and Repair of Microwave Ovens
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switch behind one of the screw holes.unless the door is securely closed.if it is open.and replacement controller or even just the main chip may be nearly as expensive as a complete new oven. only components on the primary side of the power transformer will be affected. Other than checking for bad connections and obviously bad power supply components. they will require you to open and close the door to reset their pathetic brains. also check for bad solder connections. Faulty controller or its power supply .a power surge may have damaged the electronics.on many ovens.
No response to any buttons on touchpad
There can be many causes for this behavior (or lack of behavior): Door is not closed . At this point. Of course.some models (like Sharp) have a timeout.org/sam/micfaq. :
Dead controller
The most common way that the controller circuitry can be harmed is by a power surge such as from a lightning strike. Also see the section: Getting inside a microwave oven.a power surge or random non-reproducible action of the universe may have resulted in the controller's program ending up in an infinite loop. If the controller power supply is working and there is still no sign of life (dead display and no response to buttons) the microcontroller chip or some other part may be bad. If not. Hopefully. Pull the plug for a minute or two to reset it. It could be a simple part like a capacitor or diode.this can result in the controller thinking the door is open and ignoring you. I don't know how common this practice is but have heard of it on some Sharp models. there will be no response to any buttons . then check the power supply for the controller next. Defective interlock switches . If you close the door but don't proceed to activate any functions with a couple of minutes. circuit board traces may have been vaporized (but repair may still be possible by simply jumpering across the crater).repairfaq. the transformer will need to be replaced.htm
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. a schematic of the controller board will be needed . but they would all need to be tested. there may be a fuse/thermal fuse underits outer insulation. Controller is confused . In some cases.often impossible to get . Check the primary of the power transformer . any number of other pre-existing or induced problems can result in the oven playing dead after it has been "repaired".

First. a failure of this type is extremely unlikely since power to the microwave generator passes through 2 of the 3 interlock switches. If there is any visible damage to the touchpad . Also see the section: Some of the keys on the touchpad do not function or perform the wrong action.the outer film is broken .) Much more must be enabled to actually power the magnetron so this might point more to the controller as being faulty but not always.for some reason the display portion of the controller may have been sent out to lunch by a power surge or alpha particle.
Oven runs when door is still open
WARNING: Needless to say. Defective or damage touchpad .regardless of whether a cook cycle has been selected. DO NOT operate the oven with the door open! While extremely unlikely. If this doesn't help. the cause could be a shorted triac or relay or a problem with the controller or touchpad.physical abuse is not a recommended technique for getting a microwave oven to cooperate. If both of these failed in the closed position.
http://www. (You could also note the normal sound change or slight dimming of lights that accompanies operation of the magnetron. Try pulling the plug for a minute or two . Where no such damage is evident. The only way this could really happen would be for the 'fingers' from the door that engage the interlocks to break off inside the oven keeping the interlocks engaged. Also see the section: Whacked out controller or incorrect operation. the controller would think the door was always closed. the microwave be generator could be running! For microwaves to actually be generated with the door still open would require the failure of all 3 interlock switches.if you recently power washed the oven (or even if you only use some spray cleaner). some may have gotten inside and shorted out the touchpad or controller. It woudn't be the first time. Another more benign possibility is that one or more fans are running as a result of either a defective sensor or normal operation to maintain air flow until all parts have cooled off. put a cup of water into the oven and let it run for a minute to check for heating.Notes on the Troubleshooting and Repair of Microwave Ovens
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Touchpad or controller board contaminated by overenthusiastic cleaning . In this case.htm
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.
Oven works but totally dead display
If all functions work normally including heating but the display is blank (assuming you can issue them without being able to see the display). unplug the oven for a couple of minutes to try to reset the controller. the third switch would have blown the fuse the last time the door was opened.repairfaq.it will probably need to be replaced. Check for bad connections between the display panel and the power supply and solder joints on the controller board.
Oven starts on its own as soon as door is closed
If the oven starts up as soon as the door is closed . the problem is almost certainly in the controller or its power supply.org/sam/micfaq.

The end-of-cooking cycle or keypress tone may be wailing away continuously. the cost to repair would be very high as the custom parts are likely only available from the manufacturer.Notes on the Troubleshooting and Repair of Microwave Ovens
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With everything else operational. some or all operations .possibly just a power supply but could also be the controller chip.even setting the clock . In this case. the display panel may have fractured though it would take quite a bit of violence. or be displaying in Greek. (By 'tone' I mean from the controller (not a low buzzing or humming when attempting to cook which would indicate a microwave generator power problem like a shorted magnetron).org/sam/micfaq. unless this oven has a lot of fancy features. Some ovens will not allow any actions to be performed if the door has been closed for more than a few minutes .repairfaq.
http://www. try unplugging the oven for a couple of minutes . This and the other parts are easily replaceable. The microwave generator circuits could last a long time or fail tomorrow.
Whacked out controller or incorrect operation
The following are some of the possible symptoms: All the display digits may have come on. Depending on the model. more serious damage to the door seals may have resulted as well which would be a definite hazard. for the special case where pressing START results in erratic behavios. (This could also be a shorted triac or power relay). The oven may start cooking (or at least appear to) as soon as the door is closed. you can buy a replacement (depending on size) for $100-200 so it is probably not worth fixing unless it is something relatively simple and inexpensive. If you recently cleaned the oven. Assuming this does not apply. so these should be checked. The output of the magnetron tube may decrease slightly with use but there is no particular reason to expect it to fail any time soon. it sounds like a controller problem .perhaps the controller is just confused due to a power surge. If the oven was physically abused. If the oven seems to have a mind of its own . However. The controller's program may be corrupted (unlikely) but we have no real way of diagnosing this except by exclusion of all other possibilities.may be conditional on the door interlocks being closed. see the section: Erratic behavior. A controller failure does little to predict the reliability of the rest of the oven. See the section: Some of the keys on the touchpad do not function or perform the wrong action. some liquid may have accidentally gotten inside the touchpad or even the controller circuitry (though this is less likely). Pressing buttons on the touchpad may or may not have any effect. lightning strike or the EMP from a nearby nuclear detonation because it wanted attention.running a cycle you didn't think you programmed. EEEE or FFFF. My guess is that unless you were to find some simple bad connections or an obvious problem with the controller's power supply. Pressing a button on the touchpad may result in a totally incorrect action such as entering the time resulting in the oven starting to cook.htm
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.open and close the door to reset. are you sure a previous cook cycle was not interrupted and forgotten? Try to recreate the problem using a cup of water as a load. First. However. a bad microcontroller chip is not that likely but is still a possibility.

As with intermittent problems. the main fuse may become intermittent rather than failing completely. suspect a power relay or mechanical timer (if used) with dirty or worn contacts. this sort of failure is unlikely. The filter capacitor(s) in the controller's power supply may be dried up or faulty. For mechanical timers. Microwave (RF) leakage into the electronics bay due to an faulty joint between the magnetron and the waveguide or structure failure of the magnetron may be interfering with the operation of the microcontroller. a thorough search for loose ground and other connections and bad solder joints may locate the source of the difficulty. the timing motor could be defective or require lubrication.) I only service Amana's. if erratic simply means that it doesn't heat consistently. Unless the oven was dropped or 'repaired' by an butcher. If the times and power levels appear on the display reliably but then become scrambled when entering the cook cycle or the oven behaves strangely in some other way when entering the cook cycle.org/sam/micfaq. The contacts could be dirty or worn. Prod the logic board to see if the problem comes and goes. I've only found a few that
http://www. but have serviced lot's of them over the years. suspect the controller power supply or bad connections. This may be due to a faulty part of bad connections in the controller or elsewhere. The surge or vibration of starting can jiggle the element open or closed. Check with a capacitor meter or substitute known good ones. If the oven uses a HV relay for power control. If you suspect either of these. However. or (less likely) the power surge from energizing the microwave generator or microwave (RF) leakage into the electronics bay affecting the controller. The primary relay may have dirty or burnt contacts resulting in erratic operation.repairfaq. Unfortunately. On rare occasions.htm
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. this may be defective. Reseat the flex cable connector to the touchpad. the 'problem' may disappear once the cover is removed for testing. see the section: Oven heats but power seems low or erratic.Notes on the Troubleshooting and Repair of Microwave Ovens
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Erratic behavior
There are three different situation: Whenever the oven performs unexpectedly both during setup and the cook cycle. There may be bad connections or loose lugs. there are several possibilies: The power surge caused by the cook cycle starting is resulting in changes to the settings or else the microcontroller is not interpreting them properly. this is sometimes difficult to pinpoint because unless there is obvious mechanical damage. inspect the integrety of the magnetronwaveguide joint and make sure the RF gasket is in place. See the section: Problems with internal microwave leakage.net). Where problems only occur when entering or during the cook cycle. It is easy to try replacing it!
Problems with internal microwave leakage
(From: Charles Godard (cgodard@iamerica.

and it detected leaks on everything.75 watts with the probe's styrofoam spacer placed against the door. This may require peeling it off of the front panel). The most memorable was the one with the leak that was due to the copper gasket that's between the magnetron tube and the cavity. This should permit the functions to be verified before a new touchpad is ordered. of course dropping off to near zero a few inches away. I thought I had a timer problem. or for no reason at all. For some reason that I don't remember now. liquid contamination (from overzealous cleaning.Notes on the Troubleshooting and Repair of Microwave Ovens
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leaked with my expensive leak detector. I took it apart and checked for loose solder joints and even cleaned the glass touch pad contacts. With a little bit of effort (or perhaps a lot of effort). My clue in was a spark between the waveguide and the case. If there is actual visible damage. any damage or just old age may permit spilled liquid to enter and short the sensors. Look carefully for any visible signs of damage or spills. (My worst case of actual external leakage was from a misaligned door at . Also. The symptom was that the Touch Pad timer lights and indicators would change while the unit was cooking.org/sam/micfaq. Some people have reported at least temporary improvement by simple peeling the touch pad off of the front panel and flexing it back and forth a few times. It turned out that the radiation was affecting the controller. but with the leakage from the spot welded waveguide inside the unit.net). usually only available from the original manufacturer. The touchpads often use pressure sensitive resistive elements which are supposed to be sealed. I am skeptical as this could just be a side effect of a bad connection elsewhere. the unit didn't leak. From the outside.repairfaq. it may be necessary to replace the touchpad unit. Long ago. A week or so of drying may cure these problems. (From: Matthew Sekulic (goatboy@telusplanet. when I was messing with the Controller PCB. However. for example). Our calibration meter showed a two watt leakage. this dislodges some bit of contamination. Caution: unplug the microwave generator from the controller when doing this sort of experiment!
http://www. Reseating this cable may cur a some keys dead problem. use resistors to jumper the proper contacts on the flex cable connector to simulate key presses. with the cover on. I just reformed the gasket and reseated the magnetron and that fixed the leak. the internal circuitry of the touchpad can be determined. I checked for radiation with the cover off the unit and found it extremely high. Presumably. check the snap type connector where the touchpad flex-cable plugs into the controller board. I tried one of the cheapie detectors because one of my parts supply houses suggested it.htm
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. with none escaping the outer case when attached. After that I shelled out the bucks and bought a real detector.) I have had a similar experience with a Sanyo.)
Some of the keys on the touchpad do not function or perform the wrong action
Touchpads are normally quite reliable in the grand scheme of things but can fail as a result of physical damage (your spouse threw the roast at the oven). similar symptoms. Then.

Oven heats on high setting regardless of power setting. Loud hum and/or burning smell when attempting to cook. Check for bad interlocks or interlocks that are not being properly activated. cook time. If you can locate the matrix connections for this button. This is a long shot but might work. Arcing in or above oven chamber. If there is an alternate way of activating the cook cycle. and then dry it out thoroughly. Use this to confirm the basic controller logic and interlock circuitry. try it. see the section: Erratic behavior. after all. it sounds like the controller is either not sensing the start command or refusing to cooperate for some reason . Otherwise. and would dump it otherwise. It is as though the START button is being totally ignored. Oven immediately starts to cook when door is closed. the resistance should go down dramatically (similar to the other buttons). Check that there are no missing power supply voltages for the controller and bad connection.Notes on the Troubleshooting and Repair of Microwave Ovens
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If the problem was the result of a spill into the touchpad. if there is a momentary response but then the oven shuts off. Fuse blows when closing or opening door. remove the touchpad entirely and wash it in clean water in an effort to clear out any contamination. See the section: Some of the keys on the touchpad do not function or perform the wrong action. and power setting.perhaps it thinks an interlock is open. Oven heats but shuts off randomly. replacement will probably be needed. Oven heats but power seems low or erratic. then do the same using high purity alcohol to drive out the water. sees quite a lot of action! Assuming it is not the touch pad.
No heat but otherwise normal operation
If the main power fuse is located in the primary of the high voltage transformer rather then at the line input. The START button does. including no relay action and the timer digits do not count down. Sharp Carousel IIs have a 'Minute Plus' button which will cook for one minute on HIGH. if the
http://www. pressing START does nothing. If it is also ignored. Fuse blows when microwave shuts off (during or at end of cook cycle). Or. Next confirm if possible that the START touch pad button is not itself faulty. then there may be a bad interlock or some other problem with the controller.htm
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. Fuse blows when initiating cook cycle. then the problem may indeed be a faulty START button.
Microwave oven does not respond to START button
While all other functions operate normally including clock. the clock and touchpad will work but the fuse will blow upon initiating a cook cycle.
Microwave generator problems
Failures in the microwave generator can cause various symptoms including: No heat but otherwise normal operations. However. the timer would start counting. if you have nothing to lose.repairfaq. If it works. (However.org/sam/micfaq. For example. Testing the relay or triac control signal will likely show that it is not there. Most of these are easy to diagnose and the required parts are readily available at reasonable prices.

near zero ohms. or bad connections Shorted HV diode . Open HV diode . a defective coil. A number of failures can result in the fuse NOT blowing but still no heat: Bad connections . If this is blown. It should read as a dead short . Some models may have a separate high voltage fuse. A shorted HV diode. There are other variations depending on whether the cooling fan. oven light. See the section: Testing the high voltage transformer. cooling fan. or turntable rotation
This means the controller thinks the oven is working but the microwave generator AND motors
http://www. (However.This failure may also be due to loose. It should read as a dead short . Open winding in HV transformer.see the section: Testing the high voltage diode.htm
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. or certain parts of the HV wiring would probably result in a loud hum from the HV transformer but will likely not blow the main fuse. high voltage fuses are somewhat rare on domestic ovens. A common location is at the crimp connections to the magnetron filament as they are high current and can overheat and result in no or intermittent contact. See the section: Testing the magnetron. a number of other component failures could result in no heat as well including a defective relay or triac. Open magnetron filament .some ovens have one of these in the primary circuit. Short or other fault in the magnetron .not present on most domestic ovens . Open HV capacitor .) Depending on design. Defective HV relay. interlock switch(s).
Timer and light work but no heat. See the section: Testing thermal protectors and thermal fuses. This may have dirty or burnt contacts. Test for continuity.Notes on the Troubleshooting and Repair of Microwave Ovens
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fuse has already blown there will simply be no heating action once the cook cycle is started.near zero ohms. A few models use a relay in the actual high voltage circuitry (rather than the primary) to regulate cooking power. and controller. See the section: See the section: Testing the magnetron. It may be in either connection to the HV transformer or elsewhere.org/sam/micfaq. magnetron.these may be almost anywhere in the microwave generator or the primary circuit of the HV transformer. See the section: Testing and repairing the wiring and connections.repairfaq. or deteriorated press (Fast-on) lugs for the filament connections and not an actual magnetron problem.see the section: Testing the magnetron. Test for continuity. Open thermal protector .see the section: Testing the high voltage capacitor.might blow. burnt. However.usually located on magnetron case. and so forth are located down stream of the fuse. there will be no heating but no other symptoms. Open thermal fuse . A shorted HV capacitor would likely immediately blow the fuse. Short in certain portions of the HV wiring. the HV fuse .see the section: Testing the high voltage diode.

If the oven was dropped. or a defective interlock switch may result in either consistent or erratic behavior of this type. this can happen at any time regardless of the control panel settings or whether the oven is in the cook cycle or not. Marginal door alignment. This may be erratic. Note that these symptoms are subtly different than just having no heat and eliminates the actual components of the microwave generator from suspicion in most cases. There may be a burnt odor associated with this behavior:
http://www. There is also a slight possibility that the relay or triac on the control board is not closing but those usually do not result in these particular symptoms. On some ovens. then such damage is quite likely. which might result in dangerous microwave radiation leakage. On others. and the plastic 'fingers' which operate the interlock switches as well. (From: Bonita Lee Geniac (bgen@wdl. one of the interlock switches is actually across the power line.is that a defect in the interlock switches or door alignment. if the sequence is not correct. the power line will be shorted when the other switches close.some form of which is used in virtually all microwave ovens . This will prevent the oven from being used until it is inspected and repaired. broken 'fingers' which operate the switches. The rational for this basic design . dislocated parts in the interlock mechanism. for example.htm
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.) When the timer counts down but nothing else works.repairfaq. that switch will not turn off before the other switches turn on shorting the power line. if its contacts are welded closed. warpage. Again. 99% of the time the lower door switch is bad or else the door is not closing fully and the latch hooks are not depressing the upper and lower switches. will produce a hard permanent failure. Most of the microswitches used in recent production microwaves are very poor quality and the silicone lubrication used by some of the manufacturers migrates into the switch contact area and makes the switch fail even faster than it should. and other indications of problems with the door and interlock mechanism Of course. there could be much more extensive internal damage as well. it can only happen when interrupting the cook cycle by opening the door or when initiating the cook cycle from the front panel (if the switches are in the wrong state). its mounting. the power line will be shorted blowing the fuse. The sound may originate from the HV transformer vibrating and/or from within the magnetron depending on cause. See the section: Testing and replacing of interlock switches. The cause is almost certainly related to either the door interlock switches or the door itself. As noted.net). Similarly. Look for broken or dislocated parts.
Fuse blows when closing or opening door
This means that the main fuse in the microwave (or less commonly.org/sam/micfaq. If the switches are activated in the wrong sequence due to a misaligned door.Notes on the Troubleshooting and Repair of Microwave Ovens
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aren't being powered.
Loud hum and/or burning smell when attempting to cook
A loud abnormal hum is an indication of a short somewhere. the fuse or circuit breaker for the power outlet) pops when the microwave oven door is closed or opened. Inspect the door. if the oven was dropped. occurring only 1 out of 10 times.

Remove one end of the lead from the HV capacitor to the transformer. Replace or get a new oven.see the section: Testing the high voltage transformer. 5.nz). Restart oven.repairfaq.) 1.see the section: Testing the high voltage diode.Notes on the Troubleshooting and Repair of Microwave Ovens
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Shorted HV diode . Although there will be a high current flowing in the HV transformer secondary through the HV capacitor (which is what causes the hum or buz). or wiring on the load side of the HV capacitor. the real power consumed will be reduced since the current and voltage will be out of phase (due to the series capacitor) and the power factor will be low. go to step 4. Other short resulting from frayed insulation or wires touching in the microwave generator.) (Portions from: Tony (tonyb@ramhb. Start the oven.org/sam/micfaq. it is the HV circuitry. Shorted HV transformer . 3.500 W would be reasonable. Discharge the HV capacitor again. reconnect wire and disconnect the 2 wires to the magnetron. (Not uncommon. Discharge HV capacitor! (If there is a short it is doubtful if it has any charge but never hurts to be safe). A reading on an AC line wattmeter of 300 W compared to the normal 1. Arcing within the Magnetron case (visible through ventilation holes in the bottom section) is usually an indication of a bad magnetron.co. magnetron. Note that a short on the load side of the HV capacitor will likely result in the actual wattage drawn from the power line being much lower than under normal conditions.see the section: Testing the magnetron.htm
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. Hum gone? If so. magnetron is shorted. 2. Shorted magnetron (filament to anode) or other internal fault in the magnetron . If the odor is coming from the oven chamber. 6. Hum still there? If so. The other items listed below would likely blow the main fuse but possibly not always. (The oven will run 100% without this protection for the
http://www.) 4. You have either Shorted HV capacitor. Shorted clamp diode across the HV Cap terminals (if one is present. see the section: Arcing in or above oven chamber. Short resulting from burnt on food (usually) in or around the waveguide. If it still hums you probably have a faulty HV Transformer. go to step 6. Hum Gone? If so.200 to 1. Shorted HV Diode. The following procedure will quickly identify the most likely component if the problem is not food/spills/carbon related: (Usually a loud hum that doesn't result in a blown main fuse is caused by a short in the HV diode. about 30% of microwave ovens use these).

Usually.repairfaq. There may be little plastic pins or snaps which tend to get gummed up with burnt food and may be difficult to pry off from inside the oven.org/sam/micfaq. In particular. above. and possibly melting plastic or metal. if necessary) to verify that there are no other problems (there probably are none). particularly around and inside/above the waveguide cover. Trim to fit with a pair of heavy duty scissors. The material can be obtained from places like MCM Electronics which you then cut to size with a pair of scissors or a paper cutter. you need to clean inside the waveguide above the inside top of the oven as well. Any sharp metal edges may also result in arcing or sparking. usually mica. you need to remove the outside metal cover in order to remove the waveguide cover. Clean the waveguide cover and clean inside the waveguide as well.Notes on the Troubleshooting and Repair of Microwave Ovens
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HV capacitor but it should be replaced if possible. or a paper cutter.
http://www. it may be stuck. If your oven uses a stirrer above the oven chamber (no turntable). See the sections: "SAFETY" and "Getting inside a microwave oven". The oven will work fine without it but replacement will prevent contamination of the waveguide with food vapors or splatters which can lead to more expensive damage.htm
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. the only way such damage could occur as part of the oven (not added knives or forks!) would be through physical abuse. and whatever of the cover material is remaining. First. Flashing and sparking may also result from the stirrer/fan blades contacting the metal surrounding it due to the motor/bearings becoming loose or dislodged. Then run the oven (with the waveguide cover removed. Take extra care to cover all food (which you should do anyhow) until the waveguide cover is replaced.
More on the waveguide cover and cleaning
That cover is made of an insulator transparent to microwaves. Any food that gets trapped here will eventually burn and carbonize resulting in a focal point for further arcing. If the waveguide cover is broken or damaged seriously. However.
Arcing in or above oven chamber
There is often a simple cause: Arcing in the oven chamber with a normal load (a cup of water. burnt food and carbon often make this difficult so that some disassembly will be required.) Some older Panasonic ovens have a HV reed switch and these can also short but these are rare now because of the age. often just indicates that a thorough cleaning of the oven chamber is needed. However. jot down the locations of each of the screws (they are not always all alike) and stay away from everything but the waveguide cover itself (especially the high voltage components!). inside. not a metal. All traces of carbon and burnt on food must be removed. Sometimes. for example). The result will be an uneven distribution of microwave energy and localized heating. That waveguide cover is not essential to the operation of the oven but it does prevent food from entering the waveguide and getting trapped there. the waveguide cover is designed to be removable without taking the (cabinet) cover off of the oven. arcing. completely clean below. If you do need to remove the metal cover. a sheet of replacement material is available from places like MCM Electronics. metal snips.

the gate may be defective preventing the triac from ever turning on. is that one half of the triac is bad . See the section: Testing the high voltage transformer. If one side is defective. See the section: Testing and repairing the wiring and connections. Defective HV transformer (shorted windings. Shorted wiring due to vibration or poor manufacturing quality. At least one of the interlock switches is across the power line and will blow the fuse if not activated in the correct sequence. The types of fuses used in microwave ovens are subjected to a heavy load and you may find that all that is needed is to replace the fuse with one with equivalent ratings. Another possible cause of a blown fuse is a partially bad triac. There could be an intermittent problem as well which will only show up at some random time in the future. The following can cause the fuse to blow (in approximate order of likelihood): Defective interlock switches or misaligned door. If your oven has a separate high voltage fuse somewhat rare in domestic ovens . See the section: Testing the magnetron. Recall that a triac is in effect a pair of SCRs in parallel in opposite directions. The fuses used in microwave ovens are usually ceramic 1-1/4" x 1/4" 15 or 20 A 250 V fast blow type. Defective triac (shorted or partially shorted). See the section: Testing and replacing the triac. Note that a shorted magnetron or shorted HV diode .shorted or open. Shorted HV capacitor. Shorted magnetron (filament to anode . A third. See the section: Testing the high voltage diode.org/sam/micfaq. One type of failure of a triac is for it to be totally shorted causing the oven to come on whenever the door is closed. or doesn't turn on or turn off reliably. Some ovens use a triac rather than a relay to control the main power to the high voltage transformer. See the section: Testing the high voltage capacitor.htm
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. Replace with exactly the same type and rating. However. triacs and/or door switches may always be live and may result in a blown fuse at any time when plugged in or when the door is opened or closed.Notes on the Troubleshooting and Repair of Microwave Ovens
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Fuse blows when initiating cook cycle
The fuse may only blow when actually attempting to cook but depending on design. the main fuse will blow due to transformer core saturation since the triac will act as a rectifier and transformers really do not like DC. Fuses sometimes blow for no apparent reason. Alternatively. A poorly timed power surge (as opposed to the well timed variety) could also weaken the fuse element resulting in eventual failure. Fuses also die of old age. Old age or power surge. See the sections: "Fuse blows when closing or opening door" and "Testing and replacing of interlock switches".
http://www.which you would think should blow the fuse probably will not do so because current will be limited by the impedance of the HV capacitor (assuming it is not shorted as well).it may certainly blow due to a fault in any of the HV components. Such a sound in conjunction with no heat is a likely symptom of a shorted magnetron or HV diode. there will likely be a loud hum from the HV transformer as it strains under the excess load.repairfaq. Shorted HV diode (see note below). and most interesting possibility. (but check for shorts first).see note below).

Finally. You can usually confirm the problem by setting the oven to a lower power level.) I had the exact same symptoms on my GE microwave. say "medium. a common cause that might not be obvious is that the triac used to switch power to the high voltage transformer is faulty. and excessive current which blows the fuse.00.htm
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. The only other alternative is to replace the board. (From: John Montalbano (jrmont@iquest. However. and I've seen it baffle many a repair shop. Exactly how a bad relay could result in these symptoms unless it was actually arcing and shorting is unclear. You will probably hear a 'thump!' each time the magnetron cycles on. Depending on model.00) solved the problem. but now it's gotten expensive . I replaced the triac with a $3 15 amp off-theshelf triac and it has been working for several years since.repairfaq. there is anecdotal evidence to suggest that inspecting the relay contacts and cleaning them if necessary may cure it in some cases. The triac is probably located beneath a red plastic guard on the power control board. (From: John Gallawa (microtech@gallawa.
http://www. This is an indication of a weakened triac.
Fuse blows when microwave shuts off (during or at end of cook cycle)
This could be due to a number of faults including shorting wires or defective relay. Its designation is usually Q1.00 from GE) with a new NTE56014 ($13. (From: Les Bartel lbartel@veribest. The cost used to be pretty reasonable. or SK 10265. What is probably happening is that only one half of the triac (recall that a triac is controlled for both polarities of the line voltage/current) is turning off completely resulting in DC to the HV transformer. This is a fairly common problem in GE and Hotpoint models that use this board. The following description applies directly to some GE and Hotpoint models. See the chapter: "Testing and Replacement of Components" for more information on triac testing though replacement is probably the only sure test. Drive to the triac could also be marginal but the bad triac is more likely. Modify it accordingly for your oven. Replacing the triac GE Part number WB27X5085 ($65.com). It is likely that the triac on the 'Power Control Board' is breaking down.) I have seen exactly this problem.org/sam/micfaq. install the outer cover.com). core saturation. Replace the triac (Q1) with either of the following: ECG 56010.probably about $80. However. the triac may be located on the control board or mounted directly on the chassis.) The microwave oven in my General Electric JHP65G002AD cooking center blew its 15 AMP fuse each time the timing cycle expired. and test the oven for proper operation." and heat a cup of water.net). replace the line fuse.Notes on the Troubleshooting and Repair of Microwave Ovens
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See the chapter: "Testing and Replacement of Components" for more information on this and similar problems.

there is no regulation. If you run the oven on HIGH. a failure of the controller or sensor (if you have one) could result in short cycling. or controller. Make sure the magnetron is powered continuously and it is not cycling. for low. You should see a periodic
http://www. Alternatively.check it. its failure to rotate can result in hot and cold spots. An oven that sees daily use may indeed weaken over the course of several years. You can often tell by listening for the relay clicks and/or by observing the oven light/other lights dimming as the magnetron kicks in. For 'HIGH'. A 10% drop in line voltage is likely to reduce microwave power output by more than 20%.
Oven heats but power seems low or erratic
Some considerations are how old the oven is and did the problem happen suddenly or did it just gradually weaken over the years. it is likely to be due to one of two possible causes .repairfaq. lights on the same circuit or the oven light may dim slightly when the magnetron kicks in. See the section: Testing the oven . Magnetrons.the water heating test. 50% power should result in approximately equal on and off times. However. are you sure the problem is real? Perhaps you are just a little less patient than you used to be.the microwave power should stay on continuously while it is cooking. Where a spinning paddle wheel is used to 'stir' the microwave energy (often where there is no turntable). The relay or triac may have failed in the on state. Replacements should be readily available. Check for bearing failure.kind of like slow pulse width modulation. the line voltage may be low. Therefore. you can put a microwave power indicator (NE2 neon light bulb with its leads twisted together) in the oven (with a cup of water for a load) and observe it through the window. it runs continuously. it may run 10% on and 90% off. like other vacuum tubes. it will be more difficult to diagnose as schematics for the controller are usually not readily available. can weaken with age and use. If it is cycling.Notes on the Troubleshooting and Repair of Microwave Ovens
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Oven heats on high setting regardless of power setting
Power levels in a microwave oven are controlled by cycling the microwave generator on and off with a variable duty cycle .htm
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.check if you can hear this.a faulty relay or Triac. This will probably show up with ohmmeter tests (with the oven unplugged!) but not always. broken or lose belt if direct driven. it is not really possible to inspect for correct operation with the cover removed. Perform a water heating test or try to pop a bag of popcorn using you usual time setting. However. If the problem is is the controller. etc. However. you may see an unexplained variation in cooking times. it could be something simple like a bad connection or dirty connector. Thus. Mechanical problems are also possible. The paddle is often accessible by unclipping a plastic cover above the oven cavity. can you tell if it is actually heating continuously or rather it thinks you want LOW? Many microwave ovens make a clicking sound as they use a relay to switch microwave power on and off . there may be a problem with the controller or you may unknowingly be in a low power mode . binding. If you are subject to brownouts or are running on your own generator. other settings are in between. It is unlikely that any other electronic components could change value in such a way as to significantly affect power output. When the oven always seems to be stuck at high power. There should not be any cycling on HIGH .org/sam/micfaq. Testing on HIGH will eliminate this possibility. First. Power output is quite sensitive to the AC input . Note that some are rotated by air flow from the cooling fan and require that cover to be in place to rotate.

or some sheet metal or the high voltage power transformer laminations vibrating.repairfaq. other mechanical problems. It could be bad connections in the controller or elasewhere. and thermal protector. If it is not.org/sam/micfaq. bad cooling fan (or just built up dust and grime block ventilation grilles). There could be intermittent connections to the magnetron filament.
http://www. Should it gets stuck. Test by clipping a light bulb across it or monitoring with a multimeter on AC voltage. bad thermal protector. or a controller problem. I would not suspect the magnetron or thermal problem as no cool down time is required. some other intermittent component. then the magnetron could be faulty but check for the obvious cooling problems first: blocked or dirty ventilation grill. If the magnetron was overheating. But. you would not be able to resume cooking until it cooled and the thermal protector reset. or elsewhere. Extremely high power line voltage may also result in overheating on a poorly designed or oven where the components are marginal. if even for a few seconds. gummed up or lack of lubrication. See the section: Testing thermal protectors and thermal fuses. Make sure the stirrer fan is turning normally. If resetting it allows cooking to resume immediately. fan blades hitting something. If it is not possible to resume cooking for a few minutes indicating that something needs time to cool off. HV Diode. There may be something stuck under the turntable or above the waveguide cover interfering with the stirrer. the filament opened). this is most likely either a fan or other motor vibrating on its mounts. or bad connections.
Oven makes (possibly erratic) buzzing noise when heating
Assuming operation is normal otherwise. Inspect and clean and tighten (if necessary) all connections in the microwave generator including the magnetron filament.not just a weak oven. HV capacitor. Jiggle the door to see if this will cause it to shut off. a marginal door interlock switch. This could be a faulty magnetron. If it just stopped working (i. Be sure to unplug the unit first and discharge the HV capacitor before touching anything! The thermal protector may be intermittent. everything would appear normal but there would be no heating. some models may sense this and shut down/restart.
Oven heats but shuts off randomly
Everything operates normally.. a bad motor. thermal protector. faulty controller. or bad connections. If the magnetron were shorting.htm
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. Something may have loosened up with age and use. but the oven shuts off after varying amounts of time. these would likely show up as erratic operation . there would likely be a loud hum associated with the periods where there was no heat.e.no heat at all sometimes . there could be a broken or weak belt. Determine if the magnetron cooling fan is operating by listening for its sound or looking through the ventilation opening in the back of the oven. HV transformer.Notes on the Troubleshooting and Repair of Microwave Ovens
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variation in intensity as the paddles do their job.

turntable improperly installed).it may be behind a non-removable grille requiring the removal of the cover. Oven chamber . Clean and/or replace if needed. Bad motor. When any of these do not operate properly. Therefore. at least identifying the cause is probably a good idea. You would think that something like replacing a light bulb would be trivial and self evident.always present. the most likely causes are: Gummed up lubrication/dry bearings. winding is bad but check for break at terminal which you can resolder. If open.stretch it by about 25%. if the main cooling fan is on its way out and it stops or gets stuck. Also confirm that there are no other mechanical problems (e. This is the easiest.
Oven light does not work
If the oven light no longer works. Disconnect one wire and check for continuity with an ohmmeter. 2. Unfortunately.it may be behind a mesh grill requiring a screw or snap to be removed. Test to determine if it is worn and flabby . Light bulbs may be typically located in any of 3 places: 1. no damage is likely to result.the bulb may be in a recessed compartment accessible by removing a screw or two on the back of the oven.org/sam/micfaq. Rear .Notes on the Troubleshooting and Repair of Microwave Ovens
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If the noise is caused be simple vibrations. a burned out light bulb is likely.repairfaq. The solution may be as simple as tightening a screw or weging a shim between two pieces of vibrating sheet metal. It should return to its relaxed length instantly. Convection air circulation (combo units only). Turntable. Bad connections are also possible but not that likely.
Fans or turntables that do not work
There are up to 4 motors in a microwave oven: Magnetron cooling fan . These are typically not your usual vanilla flavored appliance bulbs either.
http://www. 3. parts will overheat quite quickly at which point the oven will shut down (hopefully) and there could be damage to the magnetron or other components. Mechanical timer (on inexpensive non-touchpanel or older units).. Loose or broken belt. Clean and lubrication as needed. Check for free rotation of the affected part(s). not always so with microwave ovens. Inside . Confirm that belt is properly installed.htm
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. believe it or not.g. However.

WARNING: A microwave leakage test must be performed any time a door is removed. Replacements for a few Panasonic models are even stocked by MCM Electronics (and no doubt other places as well). replacing the screws with similar sized screws that gripped better or using filler to reconstruct or strengthen the threaded holes would be acceptable. Locate the thermostat and jumper across its terminals with power off. Where a fan only runs when the oven is hot as in a microwave/convection oven.org/sam/micfaq. Note that the opposite problem .trace wiring and check continuity (unplugged.) Here are the door disassembly instructions from the Amana service manual. (I have visions of someone using 1/2" stove bolts through the door and handle which would definitely be a bad idea).Notes on the Troubleshooting and Repair of Microwave Ovens
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Bad thermostat. PVC (pipe) cement. or even superglue (though it seems not all brands are equally effective). (From: John Gallawa (microtech@gallawa. not model cement). Remove two screws securing the latch assembly and door handle to the outer panel (this may be all that's needed to replace the handle). Make sure the surfaces to be glued are perfectly clean (remove any residual library paste if you tried that!) and provide a means of clamping the pieces until the bond sets up (adhesive tape and/or rubber bands may be all you need). Bad connections . 3. I would NOT recommend making the repair in any manner that compromises the shielding properties of the door.. one or more of the following may work: semiflexible adhesive like windshield sealer. Therefore. capacitor discharge) to motor terminals. replaced. For example. Plastic is generally tough to glue where a strong bond is needed and where the joint is subject to abuse. Consider providing some reinforcements around the joint (i. :)
What to do if the door handle breaks off
Usually this happens at the places where the handle is screwed to the door.repairfaq.htm
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. Anything that penetrates the door seal is a potential hazard likely a very small one but it is not worth the risk.a turntable and/or fan that runs after the cook cycle is completed may be normal for your oven. Plug the oven in and see if the fan now runs all the time or at least when the appropriate mode(s) are entered.com). Many others are similar: 1. Remove six screws and release 4 spring fingers that secure the choke to the outer panel. depending on the type of plastic. the thermostat or controller could also be at fault. Duco cement. plastic cement (the kind that fuses the plastic. Replacement door handles and/or entire doors may be available from the manufacturer of the oven. disassembled. plastic splints or sisters depending on your profession) for added durability. Pry out the inner door trim with a small screwdriver on the latch side of the door.e. However.
http://www. or adjusted for any reason. 2. This is a "cool-down" function designed to allow the heat to equalize or possibly added by the company's legal department to reduce the number of lawsuits due to stupidity. I would recommend staying with repairs that can be made totally externally unless there is no possibility of a change to the integrity of the door.

the function of the glass is mostly cosmetic and a small crack should not be a problem. poking something metallic through the screen would make is susceptible to microwave pickup as well. In this case. Is this safe to continue using or should I get it fixed? Will there be any radiation leakage?" So you were throwing roasts at the oven again. arcing.even after repeated attempts. if the screen is inside the glass and now broken as well. huh? :-) If the metal screen/mesh is behind and separate from the glass. or totally hardened impossible to remove carbon deposits . future failure is possible. Thus the warning not to use any metal utensils in a microwave. Therefore. damage to the inner plastic is probably not a cause for concern as that is only there to keep the screen and inside of the door glass clean. or some other form of off-white .solid or liquid . To prevent arcing and sparking.repairfaq. almond. the chance of future expensive problems will be minimized. However. Once damage occurs .more drastic action is called for: Assuming cleaning does not work on the carbon . there is no danger. However. carefully scrape it off with a blunt knife or other suitable tool.paint blisters and peels. Even if it is not actually broken at this time. clean up spills and food explosions as soon as possible. Sharp edges and hard carbon in particular creates places where electric field gradients can become great enough to cause problems. it will tend to harden and carbonize.org/sam/micfaq. And. Special microwave oven cavity paint is available but any common gloss enamel will work just as well (and costs about 1/10th as much).Notes on the Troubleshooting and Repair of Microwave Ovens
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Crack or other damage to door window
"My microwave oven has a crack in the glass of its door. any break large enough to allow something to touch the metal screen is a hazard because during cooking. sometimes splatters may find their way above the waveguide cover and cause problems above the roof of the oven chamber in the waveguide.is not cleaned up soon after the oven is used. Until you can obtain the paint. This will probably damage the paint. the oven will work fine but since the chamber is made of sheet
http://www. there could be microwave leakage. If this happens in the vicinity of the mica waveguide cover. Needless to say. Not only will it be easier. but hot spots may develop and result in possible sparking. the glass panel or entire door should be replaced. The typical color is beige. there could be shock hazard due to microwaves inducing current in the screen. In addition. Use fine sandpaper to completely smooth out the metal and feather the edges of the paint in the immediate area.
Repairing damage to the oven interior
If spilled food .just match it to your oven (if you care). Also.htm
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. it may be damaged as well. and damage to the interior paint. Use touch-up paint (with a small brush) or spray paint. the interior needs to be smooth. Not only will this be much more difficult to remove.

jumper a 50 K ohm potentiometer in place of the thermistor. Remove and test with an ohmmeter. Leaving it larger than necessary is fine as well. do paint it. So. If the waveguide cover is damaged seriously . as you lower the resistance of the pot you should see the temperature readout climb. Heat a cup of water and the candidate material on high for a couple of minutes. A convection oven which shuts down after a couple of minutes during the preheat cycle with the temperature display (if any) stuck at LOW (even though the oven is hot when opened) may have a bad thermistor temperature sensor. The heating element will be either a Calrod type (GE trade name?) which is a steel tube
http://www.about $20. Of course.repairfaq. An infinite reading means it is bad. below. it should be fine. a replacement will be required to assure continued safety with respect to minimizing microwave emissions.
Microwave/convection oven problems
In addition to the microwave components. For most ovens. It will be located centrally just above the oven ceiling duct or elsewhere in the convection air flow.htm
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. Use a suitable bit in a hand drill to make holes in the mica for the mounting screws or plastic snaps. then the problem must be the heating element. It is a two terminal device that may look like a tiny resistor or diode and may be mounted on a metal header fastened with a couple of screws. The termister will usually be accessible after removing the oven cover.org/sam/micfaq. obtain replacement material. Also see the section: Sensor problems. Possible choices include plastic or fiberglass laminate but not all materials will allow microwaves to pass without some heating . parts like doors will need to be obtained direct from the manufacturer. As a test. then either the heating element is bad (test with an ohmmeter) or the relay controlling the heating element or the controller itself is bad. Microwave oven cavity paint. If the circulating fan runs off of the same relay and it is operating. Cheaper alternatives may be possible but you would need to know the exact specifications and it is probably impossible to obtain this information. rust will set in eventually. and even some replacement doors are available from the parts suppliers listed at the end of this document. these ovens also include an air circulating fan and an electric heating element as well as a temperature sensing themister. cut to fit.Notes on the Troubleshooting and Repair of Microwave Ovens
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steel. If the convection preheat cycle never completes and the oven is cool when opened. Replacement thermistors are available from the oven manufacturer . Any of these can fail.such that it no longer will prevent splatters from entering the waveguide.check it out. During preheat. however. If the interior of the door is damaged seriously such that either it will not longer seal around the edge properly or that the mesh screening is breeched. The overtemperature protection sensor (rather than the normal temperature sensor) is shutting the oven down. The oven will then indicate READY when the simulated temperature exceeds the setpoint. it must also not have any metal coating (don't use a piece of one of those 'browning disks' :-). If the material doesn't heat up. waveguide cover mica sheets. Mica is also non-flammable which is may not be the case with other materials. Alternatives to mica which can stand the elevated temperatures in a microwave oven may also be acceptable.

Testing to determine if the controller is responding to the input from the sensor can be done in a similar manner except that access must be from inside the electronics bay while the oven is running (the probe normally plugs in inside the oven chamber).repairfaq. may result in incorrect operation (never getting past 'preheat' or not terminating a cook cycle) or in a display of 'EEEE'. It may be possible to find a replacement Nichrome coil and form it to fit. this is generally not convenient.org/sam/micfaq. Temperature probes may use a thermistor similar to one that controls the convection portion of a microwave/convection oven. the sensor and the probe cable are the primary suspects. Check for bad connections where the probe plugs in as well as broken wires inside the cable particularly near the ends where it gets flexed.com). It may be high (hundreds of K ohms) but probably should not be open. It may only be active in certain modes. it is possible that the electronic circuitry could also be affected by a damaged or defective probe unit.
http://www. 'FFFF'. Steam/humidity probes may also behave similarly. See the section: Microwave/convection oven problems for a discussion of thermistors. The circulating fan is probably driven by a belt. The problem is the temperature sensor thermostat located on the top rear of the oven. Check the fan blades for corrosion and damage. If it is loose. Replacing this open sensor will correct the problem. Inspect the belt. check your users manual. Substitute a fixed or variable resistor and see if you can get the oven to shut off (or stay on) as a function of resistance. or wiring. This is the convection temp. However.Notes on the Troubleshooting and Repair of Microwave Ovens
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enclosing a Nichrome wire coil embedded in ceramic filler or a coiled Nichrome element strung between ceramic insulators. A problem with a sensor. cracked. There should be some resistance when measuring between the signal conductors of the probe cable. or does not return to its normal length instantly after being stretched by 25% replace it.) "The 'FFFF' display is a common problem in older Panasonic convection ovens. A very low value (a few ohms or less) might indicate a short in the cable or sensor. etc. which may break or deteriorate. sensor for the correct oven temperature. controller. The best test of the probe unit is to substitute a known good one. or something similar: (From: Wilton Itamoto (witam40231@aol.htm
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." When problems develop with these automatic features. Make sure the wire gauge and length are identical. If you have never tried the probe before. CAUTION: Don't forget to put a cup of water in as a load if you are testing microwave operation.
Sensor problems
Fancier microwave or microwave/convection ovens include various probes that can be used to shut off the oven when the food is supposedly done or maintain it at a preset temperature. Of course. ERROR. Check the fan motor and fan itself for adequate lubrication. The former is probably only available from the oven manufacture. though it is worth trying an appliance parts distributor or a place like MCM electronics first.

than a bad probe unit is likely. Therefore. for those who are interested: 1 Calorie (C) will raise the temperature of 1 gram (g) of liquid water exactly 1 degree Centigrade (DegC) or 9/5 degree Fahrenheit (DegF).) 1 minute equals 60 s (but you know this!).
Or. However. Or. 1 cup is 8 fluid ounces (fl. look for corrosion or other deterioration of the socket in the oven chamber as well as bad connections. (For Avoirdupois ounces. = 236. fat chance of that!) You can skip the heavy math below and jump right to the final result if you like. a very simple test is to place a measured cup of water in the microwave from the tap and measure its temperature before and after heating for exactly 1 minute on HIGH. convection. in one minute.htm
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.184 Joules (J) or 1 J = 0.
Testing and Replacement of Components
Please see Typical Microwave Oven Electronics Bay for parts identification. a 1 kW microwave oven will raise the temperature of 1 cup of water by:
T(rise) = (60 s * 1000 J/s * 0.6 g. A couple of minutes with a cup of water and a thermometer will conclusively determine if your microwave oven is weak or you are just less patient (or the manufacturer of your frozen dinners has increased their weight sure.oz.Notes on the Troubleshooting and Repair of Microwave Ovens
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If the resistor test determines that the controller is responding.) which is 8 x 29. To account for estimated losses due to conduction. use 28.239C/J * (g * DegC)/C)/(236. I suggest using temperature rises of 57 DegC and 135 DegF. Faulty circuitry in the controller is also possible. 1 Watt (W) of power is 1 J/s or 1 kW is 1000 J/s.35 g.the water heating test
The precise number of degrees a known quantity of water increases in temperature for a known time and power level is a very accurate test of the actual useful microwave power.239 C. 1 Calorie is equal to 4. If the probe checks out or substituting a known good one makes no difference in behavior.org/sam/micfaq. and imperfect power transfer. from a Litton microwave handbook:
http://www. Scale the expected temperature rise by the ratio of the microwave (not AC line) power of your oven compared to a 1 kW unit. Therefore.6 g) = 60.oz.
Testing the oven . if your prefer Fahrenheit: 141 °F.repairfaq.6 °C.57 g/fl.
Back to Microwave Oven Repair FAQ Table of Contents.

and poke your thermometer through it. for low power tests. Thus. Any significant discrepancy between your measurements and the specified microwave power levels say more than 10 % on HIGH . If it is good but the oven makes a loud humming sound when you attempt to cook. then the test is invalid use colder water or a shorter time.may indicate a problem. the interlock switches and door alignment should be checked. It will usually be a 1" x 1-1/4" ABC ceramic type directly in-line with the Hot (black wire) of the power cord.org/sam/micfaq. (Note: if the water is boiling when it comes out . increasing the time to 2 minutes with 2 cups of water will result in more accurate measurements due to the long period pulse width power control use by microwave ovens which may have a cycle of up to 30 seconds.Notes on the Troubleshooting and Repair of Microwave Ovens
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Heat one Liter (L) of water on HIGH for 1 minute.3 times as long on 30% power as on full power.) Replace with switches having a precisely identical fit and equal or better electrical specifications (terminal configuration.
Testing and replacing of interlock switches
With the oven unplugged. current rating).repairfaq. Oven power = temperature rise in DegC multiplied by 70. Use a plastic container rather than a glass one to minimize the needed energy loss to raise its temperature by conduction from the hot water. For the ultimate in accuracy (as these things go). invert another styrofoam cup over it. suspect the magnetron or high voltage diode. Test it with an ohmmeter . If it approaches zero while opening or closing the door. (Due to conduction and convection losses as well as the time required to heat the filament of the magnetron for each on-cycle.at 100 DegC or 212 DegF. suspect problems with the interlock switches. put the water in a styrofoam cup. (You may need to disconnect one side of the transformer primary since its resistance is a fraction of an ohm. The heating effect of a microwave oven is nearly linear.) The intermediate power levels can be tested as well.the reading should be zero ohms. a cup of water should take nearly roughly twice as long to heat a specific number of degrees on 50% power or 3. There will be some losses due to convection but this should not be that significant for these short tests. or high voltage wiring.htm
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. If it is blown. put an ohmmeter across the AC input just before the interlocks (but beyond the power relay or triac if it precedes these). When removing the old switch make a note as to where each
http://www. Refer to the schematic pasted inside the cover. high voltage capacitor. the accuracies of the intermediate power level measurements may be slightly lower). See the section: Oven heats but power seems low or erratic. However.
Testing the main fuse
Where the oven is dead or mostly dead. the main fuse is the place to start: UNPLUG THE OVEN and locate and remove the main fuse. Open and close the door slowly several times there should be no significant change in resistance and it should be more than a few ohms.

DO NOT be tempted to interchange the probe and ground wire if you are using a high voltage probe on a meter with a POSITIVE input (e. magnetron is bad or HV is not working. Sensitivity will be 10 V/A. it bypasses all
http://www. or a proper microwave oven tester . however. Even professionals have been killed performing measurements of this type using proper equipment! Luckily. the magnetron is probably fine.AND KNOW HOW TO USE IT SAFELY.g. WARNING: The high voltage components inside a microwave oven are at a NEGATIVE potential with respect to the chassis. Even slamming the door really hard has been known to knock an interlock switch out of position. DON'T! This isn't like a CD player! Most of the time. Note that a shorted as well as open magnetron also results in no current. If the magnetron is shorted. If it is very low or 0. Magnetron current . With this set at a MAXIMUM of 10 percent. Make sure the new switch aligns correctly with the actuating mechanism and then check for correct electrical operation with an ohmmeter before applying power. (Another reason to stay calm after accidentally nuking that bagel for 5 minutes on HIGH!) So if there was some kind of "event" after which the microwave failed. for testing CRT HV) and no polarity switch! The ground cable doesn't have anywhere near the required insulation. current measurements can provide enough information to help make a diagnosis. Such reduced voltage tests won't identify problems that only occur at full voltage.a switch may just need to be popped back into place. no measurements of any kind on the oven while it is operating will be needed to identify and correct the problem. the voltage on the filament terminals of the magnetron should read from -150 to -250 V with respect to the chassis.Place a 10 ohm 10 watt resistor in series with the HV diode cathode and ground.org/sam/micfaq.
Making measurements inside microwave ovens
WARNING: In general. I DO NOT recommend making any sorts of measurements on the high voltage components of a live microwave oven. Measure the voltage drop across this resistor. pull the plug and discharge the HV capacitor! High voltage . here are some guidelines to a long life: WARNING: ALWAYS pull the plug and discharge the HV capacitor BEFORE doing anything inside! Never be tempted to make any changes of any kind while the oven is on .repairfaq. where this is not the case.. Check the embossed marking on the old switch . resulting in breaker tripping at the electrical service panel whenever the microwave oven door was closed. You may be temped to break out your Radio Shack DMM and start poking away inside a live microwave oven. SET EVERYTHING UP AND THEN STAND BACK and don't forget to DISCHARGE the HV capacitor after making the measurement: If it is around this range.don't depend on location as your replacement might just have a different arrangement.DON'T even think about this unless you have a proper high voltage probe or meter.Notes on the Troubleshooting and Repair of Microwave Ovens
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wire goes. A scope can also be used if it has a proper 10:1 probe as long as you aren't tempted to turn up the Variac any higher! The scope waveform should be close to a sinusoid with its positive tips at 0 V. This will be -3 to -4 VDC across the 10 ohm resistor with respect to chassis ground. I only include this section for those who really want to know the details. However.htm
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. Get the proper equipment! One thing you can do relatively safely is to connect a Variac directly to the primary of the HV transformer. Normal anode current is around 300 to 400 mA for a typical oven.not even if your meter is being consumed by 5 foot flames! First. check the interlock mechanism first .

However. it may also be possible to replace only the one that is found to be defective or make up a substitute HV cap/diode assembly from individual components if the combined unit is excessively expensive or no longer available. Connections may open up when they heat up.it should be high in at least one direction.org/sam/micfaq. Use an ohmmeter to test the diode and capacitor.) A properly conducting magnetron will load down the HV power supply. The magnetron may short out when full voltage is applied. discharge the high voltage capacitor. While connected in circuit.) I check the HV using my 30 kV HV probe with a DMM. (Again. or more. In some cases. With full conduction by the magnetron. the most likely problems are in the microwave generator. measuring between the magnetron filament connectors (either one) or at another equivalent point.Notes on the Troubleshooting and Repair of Microwave Ovens
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current to ground.
Testing the high voltage components
WARNING: First. use the higher reading). The power supply will produce 3. Don't overlook the wiring as no heat or erratic operation can result from simple bad connections! An alternative way of determining if the problem is in the control circuits (triac.ca). These may be considered to fail/no conclusion tests .) is to connect the HV transformer primary directly to a line cord and plug. but I think this is a common configuration. but the HV remains well above the 2.the resistance of a good filament is close to 0 (less than 1 ohm). HV diode.repairfaq. magnetron. the HV capacitor charges up and then there is no more current through the HV diode (but there will be an initial transient).
http://www.100 V. HV capacitor. wiring. Where the capacitor and diode are combined into one unit. the voltage remains high. (Try it in both directions.000 volts DC. wiring) or microwave generator (HV transformer. with power disconnected.) The HV at the magnetron filament is negative to ground. Test the magnetron from the filament to chassis . Weak magnetrons conduct somewhat. There could also be a wire shorting to the chassis. If the magnetron is open. and case ground. the HV drops to between 1. See the section Safe discharging of the high voltage capacitor. (From: Michael Caplan (cy173@freenet. Assuming the oven passes the above test for interlocks and door alignment. Tape the removed wire lugs to prevent shorts. Parts may test ok with no voltage applied but then fail once operated in-circuit. etc. Test the filament for continuity . (The voltages vary with design and model.htm
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. HV diode. An ohmmeter can be safely used to quickly determine if the capacitor.carleton. If it is much too high (whether fuse blows or not). If the magnetron is nonconducting. the triac (if used) may be defective.they can definitively identify parts that are bad but will not guarantee that they are good. relay.800 and 2.500 to 4. the resistance in at least one direction should be several M ohms. capacitor is shorted. or magnetron are a dead short (as well as for an open magnetron filament). it should be possible to test each component individually. depends on the circuit. but the magnitude of the change is the key. open circuit (as when the oven is first turned on and the magnetron filament/cathode is not fully heated).100 V.

at least at low voltages. It should go through the normal cycle (of course no heat) without blowing the fuse or any unusual sounds. Or. However.org/sam/micfaq. Check the water's temperature. if a lamp is plugged into the outlet strip at the same time. Now. Although there will be a high current flowing in the HV transformer secondary through the HV capacitor (likely causing a loud hum or buzz). The resistance measured across the leads of the HV diode should be greater than 10 M ohm in at least one direction when disconnected from the circuit. discharge the high voltage capacitor. It is not likely for there to be anything in between as so much heat would result that the diode would not remain that way for long. series resistor (to limit current).Notes on the Troubleshooting and Repair of Microwave Ovens
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Plug the transformer cord into a switched outlet strip which includes a fuse or circuit breaker. something in the controller or its wiring is shorted. The following is the schematic of a simple HV diode tester:
http://www. Power the oven via its line cord. Its forward voltage drop will therefore be too great (6 V or more) for a DMM to produce a definitive answer as to whether it actually works as a rectifier. This will determine proper behavior. with power disconnected. Initiate a cook cycle. switch on the HV transformer. When the cook cycle is near its end.htm
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. the outlet strip fuse will blow or circuit breaker will trip. More complete information on testing and replacing the individual components is provided in the next few sections. See the section Safe discharging of the high voltage capacitor The HV diode can fail shorted (most likely) or open.200 to 1. the oven will now heat normally. the real power consumed will be reduced since the current and voltage will be out of phase (due to the series capacitor) and the power factor will be low. Put a cup of water into the oven cavity to act as a load. However. it will likely dim significantly due to the heavy load before the fuse or breaker cuts out. switch off the outlet strip.repairfaq. If there is a problem in this case. note that the actual wattage drawn from the power line will probably be much lower than under normal conditions. The main fuse will probably not blow. If the transformer or other HV components are faulty. The result will likely be little or no detectable heat but no other symptoms.500 W would be reasonable. initiate a 1 minute cook cycle on HIGH and with the oven running. The HV diode can be tested with a DC power supply (even a wall adapter of at least 12 or 15 V output). and your multimeter. A shorted HV diode will likely result in a loud hum from the HV transformer when a cook cycle is initiated. A reading on an AC line wattmeter of 300 W compared to the normal 1. If the problem is with the triac or its drive.
Testing the high voltage diode
WARNING: First. the HV diode is composed of multiple silicon diodes in series to get the voltage rating. An open HV diode will result in AC instead of DC across the magnetron with a peak negative value (the only one that matters) about 1/2 of what it should be.

org/sam/micfaq.g. Or. These may only occur with full voltage so unless there is obvious physical damage (e. If this is the case. but no other symptoms either. and the HV capacitor.500 VAC. 1 W + o-----------/\/\---------+------------o + | __|__ HV Good: 6 to 10 V 15 VDC _\_/_ diode Shorted: 0 to 2 V | Open or reversed: 15 V | . charring between the HV transformer windings or hole burned in the waveguide). what you have is a half wave rectifier/filter formed by the HV transformer secondary. and some power is
http://www. you may get a reading in one direction (but only one) without the need for an external power supply. no heat. Discharge the high voltage capacitor. The reverse voltage across the HV diode will be equal to: 2 * 1. It may be a good idea to solder the lugs to the wires as well (though this may be overkill). A PRV of around 8 kV is actually required even for a small oven. take care not to stress them excessively which might result in bad connections now or in the future.5 A. For example. Although a shorted HV diode is usually an isolated event. Note: the lugs on your new HV diode may just be crimped onto the wire leads and not welded or soldered. Where the diode is part of the capacitor assembly. Make sure you get the polarity correct if your replacement can be installed either way. the reverse voltage goes down somewhat. If your DMM or VOM has a resistance scale operated off a battery of at least 6 V. the HV diode. Here is why: Until the magnetron heats up and starts conducting in its forward direction.800 W power line input and a HV transformer secondary of 2. discharge the high voltage capacitor.
HV diode ratings
Most replacement microwave oven diodes are rated 12 to 15 kV PRV at . it may be necessary to eliminate the other components one by one. with power disconnected.o------------------------+------------o -
The voltage drop in the forward direction should be at least 6 V with a few mA of current but may be somewhat higher (8 V or more) with a few hundred mA.Notes on the Troubleshooting and Repair of Microwave Ovens
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240 ohms. and possibly even a defective magnetron or damaged waveguide. Most HV diodes have press fit (Fast-On) or ring lugs so replacement is very straightforward.which is likely. Possible causes include a shorted HV cap. HV diodes rated at . This will probably be much much cheaper than replacing the entire assembly.5 A are adequate for most domestic microwave ovens. Putting the diode in backwards will result in positive instead of negative high voltage and. the largest of these will have a nameplate rating of around 1. While there are some losses in the HV transformer.repairfaq.
Replacing the HV diode
WARNING: First. This can easily be 6 or 7 kV or more! Once the magnetron start conducting. it is possible for failures elsewhere to have caused the diode to blow.414 * (VRMS of the HV transformer). needless to say.. arcing between windings in the HV transformer. it may be possible to just replace the diode leaving the old one unconnected (at one end) as long as the original diode isn't tied to ground inside the case.htm
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. assume for now that the diode is good if it is not shorted . See the section Safe discharging of the high voltage capacitor.

htm
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. The capacitor is usually mounted with a clamp which is easily loosened. discharge the high voltage capacitor. Sometimes.00 ohms across the terminals (and they are not bussed together on the case). 1. the capacitor is jammed into a location that requires moving some other components to extract it. with power disconnected. perhaps. check it for proper value (should be printed on the case). bad! A high resistance does not prove that the capacitor is actually functional. with power disconnected. even though calculations using Ohms law (I = P/V = 1.Notes on the Troubleshooting and Repair of Microwave Ovens
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used by the magnetron filament. Replace in reverse order. If you have a capacitance meter. To be doubly sure that your new HV diode is happy. See the section Safe discharging of the high voltage capacitor. Even this does not prove that it will not short when full voltage is applied. Yes. and light. run the oven on full power (high) for 10 minutes with two quarts of water as a load (or a roast). the cost of a generic replacement diode is small (around $3) so replacing both at the same time is usually best.600/2. An open HV capacitor will result in no heat but no other symptoms. controller. It might be warm but should not be too hot to touch.
Replacing the high voltage capacitor
WARNING: First. Unless you have the largest oven on earth. just not shorted with no voltage across it. quickly DISCHARGE THE HV CAPACITOR.600 W into the HV generator. Tighten the clamp securely but not so much as to distort the case. closer to 1/2 of the total current actually flows through the HV diode. the capacitor is definitely shorted. Where the capacitor assembly also includes the HV diode.
Testing the high voltage capacitor
WARNING: First.which may be very expensive or difficult to obtain. motors.500 or . without a shadow of a doubt. (The following assumes no internal rectifier or other circuitry except of a bleeder resistor. and then check the HV diode for overheating. it is possible to just replace the capacitor if space permits leaving the old one unconnected (at one end).64 A) would suggest that .org/sam/micfaq.repairfaq. However. if you measure 0.
http://www. See the section Safe discharging of the high voltage capacitor. due to the design of the half wave doubler circuit. Unplug the oven (while your spouse prepares the veggies). then the capacitor is positively. However. not all the power flows through the HV diode (as would be the case with a regular power supply. If it is less than 1 M ohms. you don't need to use the exact combined part . discharge the high voltage capacitor. Thus. Substitution is the only sure test beyond this. this test is probably not needed. Adjust procedures accordingly if your oven is different.5 A is not enough. Make a diagram of the precise wiring as multiple connections are often made to the capacitor terminals. Just make sure the ratings of the capacitor and diode are correct (use a generic replacement microwave oven HV diode and a microwave HV capacitor with a uF rating within 10% or so of the old one and at least equal working voltage). this still leaves.) The resistance measured across the terminals of the high voltage capacitor should be very high several M ohms for bleeder resistor. A shorted HV capacitor will blow the fuse instantly. However.

until this happened. such problems may only show up once the filament heats up and parts expand. However. However. See the section Safe discharging of the high voltage capacitor. A magnetron with an open filament will result in no heat but no other symptoms. It may be possible to determine if the magnetron filament is actually working by connecting just the filament connections to a low voltage high current supply on a Variac (e.200 to 1. A reading on an AC line wattmeter of 300 W compared to the normal 1. There is no totally definitive way to determine if a magnetron is good without actually powering it under operating conditions but the following tests will catch most problems: Magnetron filament. note that the actual wattage drawn from the power line will probably be much lower than under normal conditions. The bad connection may be internal (in which case the magnetron will need to be replaced) or external at the filament terminals (which may be repairable). a microwave oven transformer but just the filament connections). discharge the high voltage capacitor. The effect on performance in both cases would be to reduce the effective voltage across the magnetron and thus the output power.g. A magnetron with with a short between the filament/cathode and anode will likely result in a loud hum from the HV transformer and/or magnetron when the cook cycle is initiated but the main fuse will probably not blow.repairfaq. gently tap the magnetron to determine if there is an intermittent short. with power disconnected. The one at the antenna may be visible if the magnetron is removed from the oven or with a dental mirror looking into the waveguide. See the section: Comprehensive list of magnetron failure modes. A magnetron with other faults may result in a variety of symptoms including erratic or low output power or intermittent operation.org/sam/micfaq. Although there will be a high current flowing in the HV transformer secondary through the HV capacitor (likely causing a loud hum or buzz). However.htm
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. the real power consumed will be reduced since the current and voltage will be out of phase (due to the series capacitor) and the power factor will be low.Notes on the Troubleshooting and Repair of Microwave Ovens
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What if the HV diode or capacitor are leaky?
An (electrically) leaky HV diode or cap would likely fail totally in short order since it would be dissipating a lot of power. The ceramic insulators are translucent and should show a glow with a working filament. While measuring resistance from filament chassis. the oven might continue to operate and not blow a fuse. WARNING: Make sure you ONLY have the filament connected! Evidence of arcing (visible blackening around ventilation holes in base or burnt odor) usually indicates a bad magnetron. I consider these sorts of failures somewhat unlikely as the HV diode and capacitor do not generally fail half-way!
Testing the magnetron
WARNING: First..500 W would be reasonable. The resistance should be infinite from the filament connections to the case and a fraction of an ohm between the filament terminals with the wiring disconnected from the magnetron.
http://www.

) Here is a list of typical magnetron failure modes. An internal plate-cathode short may only manifest itself under the stress of high voltage during operation.try your local appliance repair shop.org/sam/micfaq. this wouldn't be an issue!) Tap the tube while measuring to check for intermittents.htm
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.) Your magnetron may still be good. however. (a) Internal plate-cathode/filament short or (b) Internal arcing.com). Therefore.tough to test since it is such a low resistance to start. Currently.
Comprehensive list of magnetron failure modes
(Portions from: John Gallawa (microtech@gallawa. Magnetron could be gassy (or up to air) and arcover internally once power is applied. If you had one. (This part is only visible with the magnetron removed from the oven).Notes on the Troubleshooting and Repair of Microwave Ovens
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Melting or other damage to the antenna cover ('bull-nose' or 'bullet') may be the result of arcing due to problems in the oven cavity or waveguide (perhaps operating with nothing in the oven) or a defective magnetron. There is no easy way to test for these possibilities other than substituting a known good magnetron. External arcing could be at the antenna or inside the filament box. this won't be the case! :) Most common magnetron failure modes: Filament could be shorted to case . This fault isn't really likely.repairfaq.there are several diameters. Anything less than infinity means the tube is bad though it could be charring due to arcing outside the vacuum in the box with the filament connections. At 2. shift position. Internal arcing will not leave any visible evidence but the damage will result in the magnetron failing totally or running with reduced output.
http://www. right. the thermal protector will shut down the oven prematurely. and short once heated. 1. (The shape doesn't matter as long as it fits tightly . Overheating might result from a broken or cracked magnet (reduced magentic field) or other internal problems. Compare with good magnetron. (Yeh. the damaged antenna cover can be pulled off and replaced from a magnetron that died of other causes . Shorts.check with ohmmeter. If a problem elsewhere has been corrected. internal shorts and loose filament connectors are probably at the top of the list. Note: Since the antenna is attached directly to one of the vanes which is part of the anode assembly.check with ohmmeter. possible blown HV fuse (but will not likely blow the main fuse). Tap the tube while measuring to check for intermittents. check directly at the magnetron terminals with both lugs pulled off. loose filament connectors (Fast-Ons) are more likely than a broken filament. While there may be some output power. The percentage of each type of failure varies. it will test as a dead short to the case on your multimeter using DC and is normal. Tap the tube while measuring to check for intermittents. Symptoms: No heat. Filament could be shorted to itself . Filament could be open . loud hum when entering cook cycle. Internal or external arcing resulting in physical damage. However.45 GHz. The filament could expand.

Symptoms: (a) Reduced or no cooking power. However. loud hum once it occurs. Be prepared for a larger spark if you use a screwdriver to discharge it! 3. If the the magnetron terminal(s) have not been burned too severely. since there is no load. We prefer cleaning up the terminal. (I suppose the transformer absorbs most of the current surge. Open filament. its RF gasket or waveguide flange. Symptoms: No heat. Occurs as cathode emission decreases from long use. such as some commercial Sharp models and most commercial and domestic Amana models. the connection(s) can usually be repaired. See note about HV capacitor in (2) above. Symptoms: No heat. Low output. Filament breakdown. (b) RF interference. Loose filament connectors (these may be repairable). such as discoloration. 5. then soldering the filament wires directly to the terminal.htm
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. The slip-on connectors can loosen. the high voltage fuse would probably blow. possible blown HV fuse. loud buzz due to arcing when entering cook cycle. whereas the result with other foods is very unsatisfactory. An intermittent filament (internal) is also possible (but not repairable).) In fact. the vacuum envelope can rupture. 7. 2. There will often also be visual symptoms at the magnetron: Signs of overheating. In the older glass-dome models.org/sam/micfaq. Usually occurs after a few minutes of normal operation. and evidence of carbon tracks or pits on magnetron terminals when the connectors are removed. 4. Symptoms: No heat or erratic heat. there are almost no failures where the magnetron causes the line fuse to blow. Symptoms: Reduced cooking power. RF interference is possible but usually only occurs if there is actual structural damage to either the magnetron. See comments about fuses in (1) above. rarely will a shorted magnetron cause the main line fuse to blow. Note: when discharging HV capacitor. possible blown HV fuse. it may end up being charged to a much higher voltage than is normal. overheat.repairfaq. Moding. 6. or its RF (feed-
http://www. some food products (with high water content) may cook normally. Occurs when magnetron oscillates in one or more undesirable frequencies. with reference to the other symptoms below. build up resistance and eventually loose contact. Symptoms: No heat.Notes on the Troubleshooting and Repair of Microwave Ovens
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In ovens equipped with fuses that monitor the high voltage system. See comments about fuses in (1) above. But.

there is only one type 'tube' (at least for any similar power range). Generic replacement magnetrons are available for the majority of microwave ovens.
Comments on replacement magnetron quality
(From John Gallawa (microtech@gallawa.com).either the users manual or a separate warranty document. 9. occasional 'snapping' sound. Insulation breakdown of the internal leads or at magnetron insulators or antenna terminal. original magnetrons may also be available from parts suppliers like MCM Electronics . magnetron overheating. With the cover in place. Same as (7a) above.45 GHz. Symptoms: Microwave leakage into electronics bay. In some cases. Cracked magnet(s).htm
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. However. RF leakage. Off frequency. Essentially.)
http://www. Symptoms: Reduced or no cooking power.repairfaq. Full coverage on the magnetron of several years is common. burning smell from magnetron. and eventually builds up around the control panel circuitry causing unusual symptoms. you may need to convince their service department that you are qualified to be poking around inside one of *their* appliances before they will consider selling one to you (too many lawyers). Check the original paperwork that came with the oven . the escaping RF energy is confined.Notes on the Troubleshooting and Repair of Microwave Ovens
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through) capacitors. Contact the manufacturer if specific instructions on how to file claims are not provided. such a replacement may be more than half the cost of a similar new oven. Physical characteristics can change and cause magnetron to oscillate at frequencies slightly higher or lower than 2. 11. Symptoms: Arcing. They will be identified as 'original' or 'genuine' along with the manufacturer and their part number. who actually does this?!). one without the gold-plated trim :-). 8. See the section: Comments on replacement magnetron quality for some recommendations. the generic variety may actually be better than the original. These will almost certainly be much less expensive than original parts. no heat. Structural failure can cause leakage from magnetron housing. If you have not sent in the warranty registration card (right. The differences are mostly mechanical.org/sam/micfaq. Going direct to the oven manufacturer will guarantee a compatible magnetron but is by far the most expensive option. In some cases. For a typical oven. loud hum. It can be very frustrating because the symptoms disappear when the oven's outer cover is removed. quality may vary.
Where to obtain replacement magnetrons
Depending on the age of your oven the magnetron may still be under warranty.at somewhat less rediculous prices. Both original and generic replacement magnetrons are available. erratic control panel behavior. a copy of the sales receipt or other proof of date of purchase may be required. In some cases (like Sears). 10.

discharge the high voltage capacitor. with power disconnected. produce both high and low end magnetrons. See the section Safe discharging of the high voltage capacitor. Do keep it (the magnets) away from your diskettes unless you want them bulk erased! As for the old one.org/sam/micfaq. though: In many cases. Of course. The studs may be removable so that the same assembly can be used with or without them. or simply no heat. and the orientation of the filament connections and cooling fins are the same. I have found it best to go to a supplier who specializes in microwave oven parts (i. these after-market tubes are actually higher in quality than the original tube. the magnetron is fastened to the waveguide with 4 nuts on studs. The cooling fins are particularly important as there must be adequate airflow from the fan for removal of the substantial waste heat .e. as well as under manufacturer brand names. The shape of the antenna terminal . See the section Safe discharging of the high voltage capacitor. the opposite can also be true. An open winding will likely result in no heat but no other symptoms. Although the magnetron is a vacuum tube.up to half of the input power to the magnetron ends up as heat. Some manufacturers.cone. a sharp blow or fall (during shipping as well if not properly packed) could shatter the filament. They sell these under a variety of specialty names. A shorted winding or short between a winding and the core/chassis in the HV transformer may result in a blown fuse. Global Micro-parts. AMI. :-)
Testing the high voltage transformer
WARNING: First. audible arcing. Usually. It is critical that the replacement magnetron be mechanically identical: this means that the mounting configuration (studs or holes and their location).
Replacing the magnetron
WARNING: First. depending on the after-market supplier. Reuse it unless your replacement magnetron comes with a new one. see the section: The magnets in dead magnetrons. The typical schematic is shown below:
http://www. When buying magnetrons from other than the manufacturer. compare it with the original. When removing it from its mounting.htm
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. Replace other components in reverse order and then reattach the filament and HV wires. such as Toshiba and Hitachi. Transfer any thermal protector to the new unit. Here's the interesting thing. as in the case of the OEM Sanyo magnetrons. overheating. mags purchased from after-market suppliers may or may not be OEM parts (there are not that many manufacturers of magnetrons in the world). QB products). These sales people are usually more knowledgeable about the magnetrons they sell.repairfaq. bull nose. loud hum. and they can help you with proper choice and application.Notes on the Troubleshooting and Repair of Microwave Ovens
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In my experience.doesn't matter. waveguide seating surface.a metal mesh ring which seals the connection against microwave leakage. which tend to fail prematurely. Magnetron replacement is generally straightforward but other assemblies like the cooling fan may need to be removed to gain access. I have seen the low-end tubes in many brand-new microwave ovens. do not lose the RF gasket . there is probably no glass in yours (unless it is quite old) so it isn't really very fragile. Make careful notes of both the wiring and mechanical relationships. with power disconnected. a burnt aroma. or square . discharge the high voltage capacitor. When you receive the replacement. However.

you need to make sure that it hasn't been damaged as well.
http://www.org/sam/micfaq. should be infinite. The only measurement easily made would be that there is no short to the chassis.5 KV RMS ohms )||( . Therefore.1 to . The resistance of the filament winding will likely be so low as not to be detectable with your multimeter. It may be possible to repair a filament winding which is shorted to the core (the only likely place) as it is only 2 or 3 turns of heavy wire. open windings (not very likely) can be located and other faults can be identified by the process of elimination. However.htm
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. However. If you have a clamp-on ammeter.5 A or MORE )||( 25 to 150 ohms )||( AC N o---------+ ||( | +-+ HV return connected to frame | | AC G o------------+---+
Disconnect terminals as required to make the following tests: The resistances of the primary should be . Check the resistance between all windings (and to the core): Filament to primary.2 ohms typical). the arcing may have been taking place to the HV winding rather than the core. if the filament winding is adjacent to the HV winding (in the same channel). An open would be an obvious failure. Testing the high voltage transformer more fully is difficult without fancy equipment. based on the way these are wound.000 V. Primary to high voltage and core should be infinite. A typical midsize might be 65 ohms.Notes on the Troubleshooting and Repair of Microwave Ovens
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+-------------------o White wire ||( Filament winding || +-------------------o White wire || || +-------------------o Red Wire AC H o---------+ ||( )||( )||( HV Winding . Typical resistance readings for the transformer HV secondary are in the 25 to 150 ohms range (depending on the power rating of the oven) from HV connection to chassis.5 to 2. Only major short circuits can be identified in the transformer with an ohmmeter since the nominal resistance of the windings is unknown. and similar fire resistant materials must be used for the repair as were present original. However.repairfaq.5 )||( 1. a winding-to-winding short would not cause enough of a resistance change to be detected with an ohmmeter unless you could compare with an identical model transformer from the same lot number. High voltage to core should be between 25 and 150 ohms as discussed above. it must be insulated for 5. and core.1 to . high voltage. See the section: Testing the HV transformer using an AC current meter.5 ohms (. may get quite hot with normal use. However. you can measure the primary current with all secondaries disconnected.

If the fuse does not blow with the secondary isolated. remove and bypass it. you have a problem elsewhere such as a defective interlock or shorted wire. If the fuse still blows. Or. here are the input current readings at various input voltages for the HV transformer from a typical mid-size microwave oven:
Input VAC Input Amps -----------------------80 . just remove the 3 secondary connections and power it through the existing wiring using the normal oven controls. unplug the oven and reconnect the primary of the HV transformer. and initiate a cook cycle. Stand well clear when you apply power! Use of a Variac is recommended but not essential.1 110 2. However. Now. Unplug the oven. Use a 3 prong cord with H and N connected to the primary and G firmly screwed to the transformer core/mounting structure. Alternatively. Assuming the fuse does not blow. power up the oven. If the fuse does not blow.0
http://www. the fuse may not actually blow (at least not immediately) but there will likely be a loud hum when the HV transformer is powered. then the problem is likely with the triac (if used). it is assumed that the fuse is blowing due to a possible short in the HV transformer.HV diode.6 100 1.
Testing the HV transformer using an AC current meter
Where the HV transformer doesn't blow a fuse but overheats or produces insufficient output. a shorted wire.test out.0 115 3.AND possibly other windings if there is a short in the transformer somewhere. Check for damaged wires that may be shorting to the chassis. the transformer is likely good and there are still problems in the high voltage components. If the other components . If your oven uses a triac. or a short in the HV wiring.000 VAC on HV terminal .htm
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.repairfaq. WARNING: Up to 3. If the fuse still blows. Also note that depending on the severity of the fault. Disconnect the primary of the HV transformer and initiate a cook cycle. magnetron . magnetron.org/sam/micfaq. Possibly something is failing only when full voltage is applied. discharge the HV capacitor.Notes on the Troubleshooting and Repair of Microwave Ovens
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Note: in the discussion below.0 120 >4. or shorted transformer. reconnect only the magnetron filament (not the HV) to the transformer and power it up again. remove the high voltage and filament connections to the transformer.3 90 . The meter's clamp needs to go around H or N but not both. then it is possible that the magnetron filament is shorted. If the fuse now blows. there may be a loud hum as the HV transformer struggles due to a fault in the HV transformer or a shorted HV diode. Repair or replace these as necessary. the transformer can be powered up to see if the primary current it draws is reasonable with no load. if the fuse still blows when the oven is plugged in (door closed to enable the interlocks). If you have a clamp-on AC ammeter. the problem is likely with the transformer. HV capacitor. this test may be useful.

If you find evidence of this: Remove the lugs and clean the terminals with fine sandpaper or a file.Notes on the Troubleshooting and Repair of Microwave Ovens
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Above about 100 VAC. Arcing may be the result of the wire scraping against a sharp sheet metal edge due to poor placement and or vibration. Label the wires before pulling off the Fast-Ons if there is any doubt as to where they go.org/sam/micfaq. burnt. cut off the old ones and replace them. discharge the high voltage capacitor. If your readings are similar to these. Check for loose. See the section Safe discharging of the high voltage capacitor. They should not be loose . Otherwise. If any connections between the lug and the wire or HV diode are at all loose. yes. And. A bit of electrical tape may be all that is needed. Microwave oven transformers are designed with as little copper as possible. note that some lugs are of the locking variety and require that you push a little tab to release them. Try not to drop either the old or new transformer on your foot!
Testing and repairing the wiring and connections
WARNING: First. and other components of the high voltage circuits for signs of arcing and excessive heating or burning. Replacement of a HV transformer is straightforward but other assemblies may be using the transformer bolts for their mounting and/or may block your way. solder it with a
http://www. more heating. discharge the high voltage capacitor. you may need to use some creativity in anchoring it and any structures that are attached to its frame.don't just sit it in place. the transformer must be secure . the transformer is likely good.
Replacing the high voltage transformer
WARNING: First.htm
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. No. Shorted turns would result in much higher current at all input voltages. However. with power disconnected. Since the magnetron filament in particular uses high current. try squeezing them a little tighter with a pair of pliers and reinstall. Try to pull off each of the lugs. HV transformer. there was also a noticeable hum (though not nearly as great as with a secondary short). and eventual failure or erratic operation. If they are not too badly deteriorated. they will still work even if they are somewhat ugly. However. See the section Safe discharging of the high voltage capacitor. weakening of the lug. the non-linear increase in current indicates that the core is saturating with no load.you should have to work at removing them. or deteriorated lugs in the filament circuit (not just the magnetron). If the lugs and their wire connections appear to be in good condition but come off their terminals easily.especially between the magnetron. with power disconnected. Inspect the wiring .repairfaq. If the replacement transformer is not mechanically identical. any resistance at the press (Fast-On) connections will result in heating. these readings do not indicate a problem.

These approaches will work as long as there is enough metal remaining for a solid connection and may permit you to salvage a magnetron or HV transformer that would otherwise need to be replaced. use a drill to make a hole in each terminal. Thermal fuses will open at a preset temperature but do not reset. Replacement of a thermal protector is very straightforward as it is almost always screwed in place with push-on lug terminals. If the resistance is more than a small fraction of an ohm. If the bulb lights up or the meter indicates approximately line voltage .
http://www. If the thermal protector is functioning properly. The new thermal fuse will probably come with lugs attached. If you suspect a bad thermal protector in the HV transformer primary. both types should read as a dead short with an ohmmeter (disconnect one terminal as there may be low resistance components or wiring which may confuse your readings).a burn from the heat will be nothing compared to the potential shock!). If you find anything suspect. You must match both the temperature and current ratings. They should reset when they cool off. Also check for bad solder connections between the terminals on the high voltage transformer and the enameled wire used for its windings. and lockwasher.the thermal protector is defective and will need to be replaced.repairfaq. Replacements are somewhat readily available. like a relay or switch.too hot to touch (but discharge the HV capacitor first . clip a 100 W light bulb or AC voltmeter across it and operate the oven.
Testing thermal protectors and thermal fuses
There may be two types of devices present in your oven: Thermal protectors are thermostats that open a set of high current contacts at a preset temperature. there should never be any voltage across it unless there is actual overheating. the contacts sometimes deteriorate. Soldering is also possible. Alternatively.htm
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. and then fasten the (tinned) wire directly (or better yet) a new ring lug to the terminal with a machine screw. However. An open triac or one that didn't respond to the gate would result in no heat and possibly other things like the fan and turntable not working as well.
Testing and replacing the triac
A triac may fail in a variety of ways: A shorted triac would result in the oven coming on as soon as the door is closed or the power being stuck on high no matter what the touchpad setting.org/sam/micfaq. They blow and need to be replaced. nut. the device is bad. At room temperature. scrape away the enamel and surface corrosion and resolder with a high wattage soldering iron or soldering gun. An overheating condition would generally be obvious as the mounting surface on which the thermal protector is located would be scorching hot when it tripped .and there is no sign of overheating .Notes on the Troubleshooting and Repair of Microwave Ovens
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high wattage soldering iron or soldering gun.

This will result in overheating as well as no or erratic operation.htm
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. Replacement will be required. The resistance of closed contacts on a relay that is in good condition should be very low . a visual examination will confirm this. welded closed. Nearly all triac failures will be shorts. An open coil is obviously defective but sometimes the break is right at the terminal connections and can be repaired easily. If you can gain access by removing the cover. Thus. or there may be other mechanical problems. A triac where one half doesn't properly turn off would result in the main fuse blowing when the cook cycle completed.probably below the measurable limits on a typical multimeter .org/sam/micfaq. A triac where one half was shorted would result in a blown fuse due to it acting as a rectifier pumping DC through the HV transformer.
http://www. If you can get at the contacts. A relay that doesn't close (due to defective contacts or a bad coil) would result in no heat and possibly other things like the fan and turntable not working as well. the coil may be shorted or the driving circuit may be defective.
Testing and replacing the power relay
A defective relay can result in a variety of symptoms: A relay with its contacts welded (stuck) closed would result in the oven coming on as soon as the door is closed or the power being stuck on high no matter what the touchpad setting. Compare your reading with the marked or specified value and/or compare with a known working relay of the same type. The relay will most likely need to be replaced if as in this case the contacts are switching any substantial power. If the voltage is low or zero. If you measure significant or erratic resistance for the closed contacts as the relay is switched or if very gentle tapping results in erratic resistance changes. Replacement is very straightforward . the relay may have an open coil. measuring across the MT1 and MT2 terminals of the triac (the power connections) should read as a high resistance with a multimeter. If the relay is totally inoperative. If the resistance is too low.possibly peculiar ways . If the voltage is correct. corroded. binding. Remove the relay from the circuit (if possible) and measure the coil resistance. dirty. or worn. A few ohms means a bad triac.Notes on the Troubleshooting and Repair of Microwave Ovens
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A triac that didn't turn off would result in the parts of the oven continuing to run even after the timer counted to zero. As noted above. test for voltage to the coil.a few milliohms. Superfine sandpaper may be used as a last resort but this is only a short term fix. some of the windings are probably shorted. triacs can fail in other . the use of contact cleaner first and a piece of paper pulled back and forth through the closed contacts may help.so substitution or bypassing may be necessary to rule out all possibilities. worn.just don't get the wires mixed up. If the relay makes a normal switching sound but does not correctly control its output connections. the contacts may be corroded.
Back to Microwave Oven Repair FAQ Table of Contents.repairfaq. the contacts are probably dirty.

0 mW/square cm
http://www. for $388. The goal is to have a meter deflection of more than 10% of it's scale while not going off scale for sake of accuracy. such a meter is a *must* for personal safety reasons as well as minimizing the risk of liability after returning them to your customers. An inexpensive meter is better than nothing but will not be as sensitive and will not allow you to quantify the amount of any leakage. as well. They prevent radiant heat sources from affecting the meter reading.. If you work on microwave ovens. They are usually made by the same companies that manufacture other service equipment. the cones are not only used as spacers.) While I don't personally recall ever having damaged a probe while checking for leakage. Use it around the door seem and ventilation holes in the cabinet.) I found an old manual for a Narda 8100B Electromagnetic Leakage Monitor. The three probes were listed as (high/low range for each):
Probe Range ----------------------------------------8120A 0.99 and an FDA approved unit (including calibration). I do know that it is possible to do so and did happen on rare occasions. Thus.repairfaq.org/sam/micfaq. etc. The Narda manual states that their probes use an antenna/thermocouples design. MCM Electronics sells an inexpensive unit suitable for quick checks on a go/no-go basis for $6.Notes on the Troubleshooting and Repair of Microwave Ovens
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Items of Interest
Microwave leakage meters
A routine test for radiation leakage should be done before returning an oven you have worked on especially if the door or magnetron/waveguide were disturbed during the repair process. While it didn't specifically mention damage to the probes. The Holaday probes that I used had 8 diodes in the tip that formed an array. In a nutshell. one should start with the highest power rated probe and work toward the lowest power rated probe (three listed in all).com). (I used to work for a manufacturer of Microwave ovens.
Comments on microwave leakage meters
(From Barry Collins (bcollins@mindspring. Note: you should also perform an electrical leakage test to assure that all case parts are securely connected to the Ground of the AC plug. I do know that the older Narda equipment was prone to such damage.). Newer designs (Holaday) claim to be more or less immune to damage resulting from placing them into high energy fields.2 mW to 2. There is a section in the Narda manual that details how to select the proper probe to measure "unknown" leakage levels. These should be available wherever you buy quality test instruments. Holaday (sp?) makes another line of detectors and those may use a thermistor array. I have confirmed that by removing the styrofoam cone from the end of a Holaday uW leakage detector's probe and then bringing its tip near a heat source (40W bulb) caused the meter to have a significant deflection. listen for alarm..htm
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. stop and replace probe. there were overtones throughout the text that implied such (watch needle. Prices and capabilities vary widely.

While it is running. The leads of the SBD are left intact and straight and act as a 1/4 wavelength dipole. Here's the circuit:
SBD <-----------------+-|<|-+-----------------> | | +-|>|-+ LED
The LED is soldered close to SBD using as short of leads as possible (being careful not to ruin either part with too much heat). You can buy these detectors quite cheaply.uk).) A very simple design I saw somewhere (Electronics World.demon. slowly sweep the tester around the door seal.000 watt plus models.0 mw/square cm in consumers home As you no doubt know.edu). but I believe that the maximum leakages we were allowed by the governmental agency were: Less than 2. (Note that the diodes are connected anode to cathode.) I saw an article about it in Modern Electronics in the early eighties. It is simply a Schottky Barrier Diode (SBD) and an LED wired together.ucar. especially on the newer 1. Put a large container of water (>=2 cups) in the microwave and run it on HIGH for 2 minutes. (From: Ren Tescher (ren@rap.Notes on the Troubleshooting and Repair of Microwave Ovens
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8121A 8122A
2.org/sam/micfaq.0 mW/square cm 20.co.0 mW/square cm
This is from memory. hinges and door latch.) I then taped/glued it 1 1/2 and perpendicular from the end of a popsicle stick (this gives it a 'standoff' distance). with a hole cut in the oven (in reference to those who want to modify one see the section: Microwave ovens for non-standard applications --. Damage would occur where one intentionally held the lower power rated probe in the strong field until the thermocouple (or thermistor?) overheated. Determining by how much is why you pay the big bucks for a real leakage meter! WARNING: These are no substitute for a properly calibrated commercial unit! (From: Leon Heller (leon@lfheller.htm
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. not cathode to cathode.repairfaq. I think one measured the voltage across the diode via a resistor and capacitor smoothing arrangement using a 50 uA meter.0 mw/square cm leaving the warehouse Less than 5. All microwave ovens leak to some extent.0 mW to 200. You may have to
http://www. the density can easily reach several times these numbers. their utility is somewhat limited.
Simple microwave leak detectors
Since these do not really provide an absolute measurement.sam).0 mw/square cm off of our assembly line Less than 3.0 mW to 20. probably) consisted of a half-wave dipole with a Shottky diode detector between the two elements.

. The commercial tester had three ranges and the most sensitive range was divided into 3 color bands. Adjust the voltage across the bulb so that it's just barely glowing. the SBD will rectify the waves.duke.
How safe is a repaired microwave oven?
So you fixed up Aunt Minnie's Radarange or picked up a microwave at a yard sale or scavenged one off the curb.. Put the bulb on the end of a line cord and plug. However. there should be no leakage from there. How safe is it to use (assuming you evicted the cockroaches)? As long as there is no serious damage to the door (a 6 inch hole would quality as serious damage) and the door fits square. The NE-2 size is a good one. Wave the bulb around the door gaskets and if it gets brighter when the oven is turned on. Typically the LED just flickers. Small leaks may be remedied by adjusting or cleaning the door and hinges and/or by distance (square law= doubling the distance quarters the power).
http://www.sam). truly horrible mess of decayed burnt-on food. (From: James P. I built 10 of these at home and then compared them to the commercial tester we had at work. especially if the tester still glows if it is pulled beyond the 1-1/2 inch standoff distance to 3 inches. it should be properly sealed.they all came from the bargain bin at Radio shacks. The most important considerations are the door and door seal. Meyer (jimbob@acpub. Adding this onto a neon circuit tester is one option and will provide an insulated housing as well. Plug the whole thing into an AC outlet. contrary to the old wives tale of not looking through the window while it's cooking.around $8 .repairfaq. Any leaking uwaves will be picked up by the dipole 'antenna'. around the hinge/latch areas. green. red. The home-built testers all 'fired' at some point in the 'yellow' range. A solid glow would indicate excessive leakage. Large leaks . Make the divider network resistance large enough to limit the current through the bulb to just a couple of mA. and when sufficient rectified voltage has built up. (US law allows increased leakage as the oven ages). or a thriving community of cockroaches inside.will not be as sensitive or accurate as the $500 variety by may provide some peace of mind. they may indicate dangerous leakage even when your oven is within acceptable limits. (The screen really is a very good microwave shield --. the LED will light up.trash the oven. yellow. as noted below. An inexpensive leakage tester . You may notice that no radiation leaks through the viewing window.org/sam/micfaq.htm
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.. Make sure the metal cover has all its fingers engaged around the front (though with a properly installed magnetron. You may even be able to use it to find wires behind drywall in your house.Notes on the Troubleshooting and Repair of Microwave Ovens
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dim the lights to see if the LED lights up. The only problem you could find was a blown fuse. As long as the waveguide is tightly mounted and undamaged. there should be minimal microwave leakage into the electronics bay). Use some resistors to make a voltage divider for 115 VAC to feed the bulb. INSULATE everything completely.edu). then you have located a leak. I attribute the variances within the yellow (caution) range to individual characteristics of the diodes . The bulb detector can be very sensitive.) Get a small neon bulb.

For a single cup of tea. gastronomic preferences aside.htm
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. you are not heating the surrounding countryside as the microwaves only affects what you are cooking and not the container or oven cavity itself and you are more likely to only load the amount of food you expect to be eating. However. While they definitely serve a purpose. Size was reduced and the number of fins were reduced.
Efficiency of microwave ovens
The efficiency of an electric heating element is 100% . The magnetrons were cost reduced in a similar fashion.repairfaq. The light bulb and controller also use small amounts of power.even if you could distort its anatomy enough to fit the typical mid-size microwave!
Microwave oven design and cost reduction
(From Barry Collins (bcollins@mindspring.) Those inexpensive hand held meters (from Radio Shack.com). While the magnetron tube itself may have an efficiency rating of 75%. Models equipped with turn-table models compensate for this by breaking up the SWR as the food revolves. they have caused a more than a few people to unnecessarily fear microwave ovens over the years. This was all done in the name of cost reduction to remain competitive.45).
http://www. cooler running.com). you won't heat just *1 cup* of tea but more likely 2 or 3 just to be sure you have enough! A microwave oven is not likely to be more than 60% efficient . I'd almost hesitate to hope the Government would have mandated an efficiency. non-cost reduced. Also.Notes on the Troubleshooting and Repair of Microwave Ovens
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(From Barry Collins (bcollins@mindspring. Not to mention it has an older design. But then the cost went from $300 to $149 while life went from 10 years-plus to 5 years or less and they became disposable items. The thing that I found disturbing about microwave oven design was the trends to go with hotter an hotter insulation classes on the components used in them. The microwaves are designed for the most part to work optimally with an average load. the microwave oven may use 1/10th the energy of a typical electric cooktop element to bring it to a boil! Therefore. etc. more efficient magnetron (that cost $13. The original transformers were class H while the newer ones are now class N. using an electric stove to heat 1 cup of tea may result in much wasted energy as the element and pot must be heated as well and there are losses due to convection and conduction to the surrounding environment. placement of food items affect the SWR. However. What one hopes for is to deliver all the power from the magnetron into the food and not have a high SWR reflect back into the magnetron and burn it out.org/sam/micfaq. and turntable motor (if used).period.00 instead of $9. there are losses in the high voltage transformer. I personally know that it had one of the lowest SWRs available at the time. Size. I just changed jobs from working for a company that made gas ranges. In addition. That's one area.. The windings AWG got smaller and the temperature rise went up accordingly.possibly as low as 50 percent or even less. My oven has a stirrer fan design and has been working for going on 18 years now without the first hint of a problem (maybe a little less power). the waveform applied to the magnetron by the half wave doubler circuit is not ideal for maximum efficiency. These all add up to a significant overhead. However. shape. cooling fans. I'd highly recommend anyone with gas heat or appliances to purchase a quality CO detector. Furthermore. it makes sense to use a microwave oven for small short tasks where the losses of an electric or gas oven or cooktop would dominate. Their temperature went up while their efficiency went down.) can give very inaccurate readings.) Microwave oven design is a black art. but not one of those inexpensive type that go off whenever there is a thermal inversion of smog a city. a conventional oven is better suited for that 20 pound turkey . CO detectors caused similar panic among users of the appliances.

Examples would be wrapping foil around the wingtips of a whole chicken or whole turkey.. Older microwave ovens with used glass magnetrons were perhaps more susceptible to these disasters (all modern overs use magnetrons with ceramic construction but I really don't know how much this matters) but it's still a good idea to avoid running a microwave empty. Safe distances are uncertain and are usually less if the metal objects are small and a large amount of food or water is exposed. This is usually OK. adding metal objects change the microwave reflection pattern and might possibly unfavorably change things.htm
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. They don't need preheating! :)
More on metal in the microwave
(From: Don Klipstein (don@Misty. You will note that some ovens come with metal fixtures in addition to the oven walls themselves (e. Sharp convection/microwave combo).org/sam/micfaq. this is probably not likely to damage the oven. It is even not too good to run a microwave empty. you should be OK. arc. or create other fireworks. Having absolutely nothing in the oven chamber or just metal is the potentially more likely damaging situation for the magnetron as you are dumping several hundred W to over a kW of power into a reflective cavity with no load.
Burnt smell from oven .after incident
http://www. they just reflect around the oven and get back to the magnetron tube. The walls of the main cooking chamber are metal. If the added metal does not interfere with microwaves mainly getting from the tube to the target food or water and being absorbed. If you add metal objects in a manner safe for the tube. A plain glass bottle if ice-cold stuff might possibly break from thermal shock when heated. This may be bad for the tube. you could end up with a meltdown inside the waveguide requiring replacement of various expensive components including the magnetron. If you have exposed food or water. or a bottle of liquid (on its side) with a metal lid with liquid contacting much of the lid. In the event the microwave runs empty OK. and in an unpredictable manner. but any metal lid on a bottle largely full of microwave-absorbing stuff should not present a problem especially if the bottle is on its side so that stuff is contacting or very nearly contacting much of the lid. there is another concern. Sharp edges and points create strong field gradients which tend to spark. the magnetron should be OK. Otherwise. the tube should not mind some stray metal too much. With some food in the oven to absorb the power.g. try to keep these at lease a half inch (a bit over a cm. If any metal object has major contact with a microwave absorbing food target and such target is still heavily exposed.com).) from the walls to avoid arcing.Notes on the Troubleshooting and Repair of Microwave Ovens
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Problems with running a microwave oven with metal inside or totally empty
Metal in microwave ovens may or may not be a problem depending on the specific situation. Metal objects close to other metal objects or to the walls of the cooking chamber may arc to these. Any arcing is generally not a good thing. Just avoid unrelated problems due to major temperature change of anything in contact with a non-heat-rated glass container. you need exposed water or food to absorb the microwaves. Even if the tube does not mind.) Mainly.repairfaq. In the worst case.

especially considering that microwave ovens are usually situated near grounded appliances like ranges and normal ovens and wet areas like kitchen sinks. so I don't know if it was neglect or inappropriate use.Notes on the Troubleshooting and Repair of Microwave Ovens
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"My daughter tried to heat up one of those 'soup in a box' containers and it burned actually charred. nothing in the oven has actually sustained any damage. Many are not. there are many passages where the air would normally circulate in convection mode which will be coated even if the oven was used in microwave mode. Only about 50 to 60% of the electricity used by a microwave oven actually gets turned into microwaves. With a combination convection and microwave oven especially. Therefore. Touching this may result in a shock or worse. the odor may persist since the smoke can penetrate to places you cannot access for cleaning. Convection ovens have heating elements which are similar energy hogs. due to the highly inductive nature of the high voltage transformer. do not put your refrigerator on the same circuit!
Microwave ovens and GFCIs
A Ground Fault Circuit Interrupter (GFCI) protects people from shocks should a situation develop where an accessible part of an appliance should short to a live wire. that even after the incident described still performs well. I wasn't home at the time. it will not hurt to have a GFCI as well. it is theoretically possible for the entire high voltage to appear on the metal case should certain internal connections come loose.or at random times. However. Check with a circuit tester to make sure your 3 prong outlet is correctly wired. If possible. However. a 700 W oven will actually use up to 1400 W of power . If this doesn't help enough. this usually does not indicate any problem. You may have to do this several times to get all of the sticky film left behind. this will at most blow a fuse. I would expect that the smell will decrease and eventually go away. remove the waveguide cover and clean it and as best as possible the accessible part of the waveguide. Plug the oven into a properly grounded circuit not on a GFCI. With a properly grounded outlet. However.maybe even help. A GFCI detects any difference between the currents in the Hot and Neutral wires and shuts off the power should this difference exceed a few mA.nearly an entire 15 Amp circuit.repairfaq.
Can a microwave oven be built into (or hung under) a cabinet?
http://www. A GFCI is not needed with a properly grounded microwave oven as any such fault will blow a fuse or trip a circuit breaker. :)
Microwave ovens and grounded dedicated circuits
A microwave oven should be used only on a properly wired 3 wire grounded circuit. At least. I have a Sharp Convection/Microwave. There is a very important safety reason for this requirement: the return for the high voltage is through the chassis. Most likely. a shocking (or worse) situation could develop . It won't hurt . However. smoke may have gotten into the waveguide above the oven chamber.htm
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. Some have suggested boiling a cup of lemon scented water or vinegar to help speed things along. What do you recommend. While unlikely. A dedicated circuit is desirable since microwave ovens are significant users of power. but the lasting effect is that there is a strong odor. In most cases. However. similar to that which you smell after a fire that I cannot seem to get rid of.org/sam/micfaq." Start by cleaning the interior of the oven thoroughly with mild detergent and water. The rest is wasted as heat. with some combinations of oven design and your particular wiring. nuisance tripping of the GFCI may occur when you attempt to cook anything . with the case floating. Install one if it is not grounded.

etc). You will need a heavy and expensive step down or step up transformer which will likely cost as much as a new microwave oven. conformal cabinet. screws. and you pivot the oven up into place. expansion bolts. check with your friendly township inspector! There are special (likely highly overpriced) models available for this type of mounting. enclosed. It comes with a template to make this easy. some GE units have a hinged front panel .not having to remove the entire unit to check or change a fuse! For example. for microwave ovens in particular. Furthermore. Sell the oven before you leave and buy a new one at your destination.e. not just the sheetrock!). (From: Roy Smith (roy@popmail. Low cost transformers or international voltage adapters will not work.the microwave weighs quite a bit and must endure a fair amount of abuse from heavy casseroles and the inevitable door yanking/slamming! Note that one of the advantages of buying a microwave oven designed for under cabinet or wall mounting is that it may provide convenient access for servicing from the front .htm
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.) I've installed a GE over-the-range microwave.nyu.) Providing adequate structural support so the microwave doesn't end up in the soup. affects output power. the major issues are: Providing adequate air flow through its ventilation grill which is usually located in the rear. In this case I would suggest contacting the manufacturer of the oven for specific requirements.edu). before doing any demolition. Models designed as over-the-range or combined microwave and exhaust fan units mount via a massive plate fastened securely into the wall structure (screwed directly to the studs.repairfaq. my recommendation would be to build a shelf rather than a totally sealed. They may additionally be bolted into the cabinet above but this will not (or should not) be the sole means of support. To use a normal microwave. (A convection/microwave can get quite hot and have ventilation in other places. This would not be possible where a countertop oven is used in a permanent installation. So. Due to the way the high voltage power supply works in a microwave oven. etc you can get to work (i. It can have sides and a top as long as you leave a couple of inches all around. wherever you can find studs.remove a couple of screws and most of the internal components can be accessed for service. The rear-bottom edge of the oven then clips onto the backplate to form a kind of hinge.
http://www. Local building codes may specify when and if this approach can be used.
Taking a microwave oven oversees (or vice versa)
Microwave ovens are high power appliances. There are two long bolts that run the depth of the oven near the top which you use to complete the attachment of the oven to the backplate.cabinetry and/or drywall may not be up to the task. :-( These are HEAVY appliances . There is a backplate which you attach to the wall with whatever combination of lag bolts. It really was quite straight-forward.org/sam/micfaq. the HV capacitor is in series with the magnetron and thus its impedance. Just make sure it is securely supported .Notes on the Troubleshooting and Repair of Microwave Ovens
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Assuming it is a regular microwave and not a convection/microwave combo. This will result in a microwave oven that is much more easily serviced should the need arise and replaced in the future with a model that is not quite identical. line frequency may make a difference. which depends on line frequency. You then bolt it into the cabinet above it for additional security.med.

it may be possible to replace the HV capacitor with one that has about 5/6 the uF value.perhaps terminally .Chassis Ground Bridge Rectifier
http://www. The line voltage could be reduced by a small amount to compensate. This self-test is usually accessed by pressing a couple of keys on the touch pad. Some microwaves have this information tucked in a pocket or hidden somewhere behind panels. You can usually test things like keys. This means that the microwave output is pulsing at both 60 Hz and the frequency of the inverter!
Inverter Transformer Magnetron o H o----+---|>|------+--------+-------+ +--------------------------+ ~| |+ _|_ Drive )::( Filament 1T #18 | +---|<|---+ | --25T ):: +--------------+------+ | 115 VAC | | | #12 ):: HV Cap | +-|----|-+ +---|>|---|--+ +-------+ :: +-------||-----+ | |_ _| | | | | ::( .2 _|_ (Except for filament.018 uF | | \/ | N o----+---|<|---+ Drive |/ C ::( 2. Going from 50 Hz to 60 Hz at the same line voltage may slightly increase output cooking power (and heating of the magnetron).net). Going from 60 Hz to 50 Hz may slightly decrease output power and possibly increase heating of the HV transformer due to core losses.htm
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. See High Voltage Inverter Power Supply from Sharp Microwave Oven. at the same or higher operating voltage. # turns estimated) o H1 . The digital clock and timer will likely run slow or fast if the line frequency changes as they usually use the power line for reference.repairfaq. This is best done with a buck/boost transformer rated for the maximum current input to the microwave oven (usually 15 A). these may run hot even at the correct line frequency of 60 Hz.in many ways similar to the deflection/HV flyback power supply of a TV or monitor. Of course.
High frequency inverter type HV power supplies
While the vast majority of microwave ovens .use minor variations on the tried and trusted half wave doubler circuit.perhaps every single one you will ever see .) Some microwave ovens have a self-test feature. Even with no load. Alternatively. Check the manual for any self-test info. this may partially make up for your change in output power! :-)
Microwave oven test-mode
(From Mark Paladino (paladino@frontiernet.though this is not likely. switches controller etc. Using a slightly lower line voltage will reduce the heating but will further decrease the cooking power. a few models have been designed using solid state high frequency inverters .400 V __|__ | ___ | ~ |o---| Chopper ::( HV _\_/_ +----|:--+ (Interlocks and | |\ E ::( 250T | HV |'--> fuses/protectors | | ::( #26 Sense | diode | uWaves not shown) +-----------+ +--+---/\/\----+---------+ o | 1.Notes on the Troubleshooting and Repair of Microwave Ovens
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High voltage transformer core saturation may also be a problem. A typical circuit (from a Sharp microwave oven) uses full wave rectified but mostly unfiltered pulsating DC as the power to a large ferrite inverter transformer which sort of looks like a flyback on steroids. So going to 50 Hz would make it worse .org/sam/micfaq.

heavy transformers can smash your feet and sharp sheet metal can cut flesh. in fact. It is not known whether power levels in the oven from which this particular inverter unit came were set by the normal long cycle pulse width modulation or by control over a much shorter time scale. as with a switchmode power supply (which is what these really are) there could be multiple faults which would result in immediate failure or long term reliability problems if all bad parts were not located.htm
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. Panasonic has a several models like this. Compared to the simplicity of the common half wave doubler. Except for obvious problems like a tired fuse. However. QM50HJ-H.org/sam/micfaq. Possibly more have jumped off. These usually operate on 115 VAC but some may use low voltage DC. the blurb for the current line of Panasonic Genius(tm) inverter microwave ovens does boast about providing actual power continuously at each setting. The principle advantages claimed by the manufacturer are more even cooking and less overcooking of edges. other companies certainly aren't jumping on the bandwagon. a replacement module would likely cost as much as a new oven! This may simply be a situation where a high tech solution might not have been the best approach. Schematics are not likely available either. I would counter that anything unfamiliar can be of immense educational value to children of all ages. And. 01AA2. before you cannibalize your old oven. Perhaps. there are. Controller and touchpad .Notes on the Troubleshooting and Repair of Microwave Ovens
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The chopper transistor is marked: Mitsubishi. component level troubleshooting and repair would be too time consuming. an investigation of the inside of a deceased microwave oven can be very interesting. Another major advantage reduced weight . it isn't at all surprising why these never caught on (what is diagramed above includes perhaps 1/10th the actual number of components in a typical inverter module. relay and/or triac control of the AC power. this was yet another situation where the Marketing department needed something new and improved! But if it was a "must have". However.repairfaq. I don't know if any other manufacturers (including Sharp) still do. And. With appropriate supervision. possibly 8 years ago (that would be in 1996). some useful devices inside: Motors . :)
Dangerous (or useful) parts in a dead microwave oven?
A microwave oven with its power cord cut or removed AND its high voltage capacitor safely discharged is an inanimate object. or by pulse width modulation of the high frequency power. The high frequency inverter approach would not seem to provide any important benefits in terms of functionality or efficiency yet created many more possible opportunities for failure. Furthermore. There are no particularly hazardous parts inside. The microwave distribution mechanism is at least as important in this regard.cooling fan and turntable (if used). Of course. the magnets in the magnetron may erase your diskettes or mess up the colors on your TV. Some may feel there is nothing of interest inside a microwave oven.digital timer. which can be seen in the photo). They can easily be adapted to other uses. consider that many of the parts are interchangeable and may be useful should your *new* oven ever need repair! For the hobbiest.is somewhat irrelevant in a microwave oven. It is a LARGE NPN type on a LARGE heatsink. I acquired the Sharp unit at least 5 years ago. :-) Note the similarity between the normal half wave doubler circuit and this output configuration! Base drive to the chopper transistor is provided by some relatively complex control circuitry using two additional sets of windings on the inverter transformer (not shown) for feedback and other functions in addition to current monitoring via the 'Sense' resistor in the transformer return. See the
http://www.

HV rectifier (12 to 15 kV PRV. Not only can they pinch flesh (yes. They may need cleaning. diskettes.200 to 7. diskettes. they are that powerful) but they will suck all the bits right off your tapes. up to 1. These can be removed without extensive disassembly and make really nice toys but should be handled with care.
The magnets in dead magnetrons
The dead magnetron you just replaced is fairly harmless.3 or more high current microswitches.500 VAC (4. fuse holder.there are some nifty powerful magnets as part of the assembly.Typical HV transformer (1.500 to 2.500 to 2.000 VDC). The ferrite is
http://www. other miscellaneous parts. See the section: The magnets in dead magnetrons. CAUTION: the sheet metal fins may be sharp! The magnets can now be pulled off. Heavy duty power cord. and HV capacitor (approximately 1 uF. and color computer monitors and TVs.org/sam/micfaq. Keep the magnets a safe distance away from any magnetic media including what might be in your back pocket. There is no residual radiation but it does contains a pair of powerful ferrite ring magnets. There is a slight chance that the coating on the filament is poisonous so don't take chances. Remove the cover over the box where the filament connections are located. 0. (The thick copper coils are RFI chokes and prevent any microwave energy from escaping via the filament circuit. You don't need to get inside to remove the magnets.repairfaq.Notes on the Troubleshooting and Repair of Microwave Ovens
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section: Using the control panel from defunct microwave oven as an electronic timer.500 VRMS. thermal protector. Cut the thick copper connections to the filament near the tube itself. This usually requires peeling off the sheet metal around the edges. mechanical wrist watches. Interlock switches . Dangerous microwave leakage is possible. Take appropriate precautions to protect your credit cards.5 A). If you do want to save the magnets: Disassemble the magnetron assembly as follows: Remove the top portion of the magnetron .5 A). 0.) Spread the frame apart just a bit and lift out the tube with heat sink fins.htm
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. Paint the magnets with plastic enamel or coat them with the stuff used on tool handles to reduce their tendency to chip. DOUBLE WARNING: Do not even think about powering the magnetron once you have removed any parts or altered anything mechanical in the oven. High voltage components (VERY DANGEROUS if powered) . Magnetron .it is either fastened with screws or some metal tabs which are easily bent out of the way. :-( The magnetron tube itself can be disassembled by grinding off the welds around the edges of the large cylinder or cutting around it outer edge near one end with a hack saw but it takes quite a bit of curiosity to make this a worthwhile exercise. and mechanical wristwatches. The chips are as magnetic as the overall magnet. and credit cards.

However.repairfaq.htm
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. DON'T bypass or disable any door interlocks. the triac is NOT phase angle controlled .just switched on or off. Be careful when disconnecting the touchpanel as the printed flex cable is fragile. This would result in a very non-linear relationship between on-time and power as the cycle became shorter and shorter. Store the magnets in a box packed in the center of another box with at least 4 inches on all sides.org/sam/micfaq.
Precise control of microwave oven power
For heating a casserole. If it uses a triac. these magnets can be used to demonstrate many fascinating principles of magnetism.a high power light dimmer or motor speed control might even work.modern microwave oven. However. (For safety. Have fun but be careful. It should be possible to put a Variac (variable autotransformer) on the input to the high voltage transformer . My guess is that this would be between 60 and 80 percent and full voltage from the Variac will result in 0 to 100 percent of cooking power (the magnetron is a non-linear device . the touchpanel (membrane touchpad) needs to be peeled off of the front plastic panel or the entire assembly can be removed intact.Notes on the Troubleshooting and Repair of Microwave Ovens
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basically a ceramic and fragile. a triac or solid state relay can be turned on and off at the peaks of the AC (to minimize inrush) similar to the pulse width modulation that is normally used for the oven . Clearly mark: powerful magnets with appropriate warnings.
Using the control panel from defunct microwave oven as an electronic timer
It is usually possible to remove just the touchpad and controller board to use as a stand-alone timer with a switched output.order of a 30 second cycle if this matters. Just cycling faster (without any other modifications is not the answer). this approach may be the adequate. this results in unsatisfactory results. One problem is that the filament of the magnetron is turned on and off as well. Having said that. Where manual control is all that is needed. for other purposes. This could easily be computer controlled with feedback from a temperature
http://www.there is a threshold voltage below which no output is generated). the 10 to 30 second cycle time typically used for microwave oven pulse width heat control is fine. With many models.but at a much higher frequency.between the controller and HV primary. then instantaneous control of power should be possible using a Variac on the HV transformer primary or a phase control scheme using a triac . This question was posed by someone who wanted to modify the circuitry to their microwave oven to provide continuous control and a constant heating rate. Smack them too hard and they will shatter. DON'T attach it externally. Alternatively.) The power to the filament will still be affected but there will be a range over which continuous control will be possible. If the filament were put on its own transformer (with appropriate insulation ratings). Also see the section: Magnetron construction . The output will control a 10-15 A AC load using its built in relay or triac (though these may be mounted separately in the oven). there will be a lag as the filament heats and cools. and make sure the cooling fan is always powered from the full line voltage. Take care not to get your skin between the magnets when you bring them together since the attractive force when nearly touching is substantial. Note that power on a microwave oven is regulated by slow pulse width modulation .

does it run Lotus??? :-) --." etc. you tell it what you are defrosting." "baked potato. While that sounds like overkill. you haven't seen it all.including cooling fans ." "beverage. or Reheat. etc. choose a number from the list. For things like CompuDefrost. A typical session goes like this:
Button Pressed ---------------CompuCook 1 4 Screen Output -----------------------------Enter Food Category Baked Potato. softening butter or chocolate. (From: Steve Dropkin (sdropkin@isd. (Same argument I
http://www. There is even an 'on-line' help feature. a microwave with a robot arm to retrieve the food from your fridge or freezer! But wait. If the little word HELP lights up.Notes on the Troubleshooting and Repair of Microwave Ovens
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sensor." "waffles. etc." "hot dog. :) (From: Dave Marulli (marulli@rdcs. takes you step-by-step through anything the oven can do." "frozen dinner.org/sam/micfaq.net). It will turn on for a while. will contact a recipe database at the Web site for the product to determine exactly how to optimally overcook it and turn it into rubber. close the door hit start and it continues until it's time for you to do some thing else. etc.kodak.
Has technology gone too far?
Don't you just hate it when your kitchen appliances have the highest IQ in the household? What more could you want? Maybe. and includes 600 recipes (!).htm
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. It even has the obligatory POPCORN button! Another neat feature is that you can hold the start button on without setting any time and it will stay on for as long as you hold the button. They looked just way too busy. enter the 'quantity'. This is great for melting cheese. DO NOT COVER. You WILL see ovens (if they don't exist already) that with the help of a barcode or Dallas ID chip on the frozen package or food container. Same idea for CompuReHeat: Tell it how many slices of pizza or bowls of pasta you want to reheat. hit start and it picks the time and power level. There is a list of common items that you might Defrost. then beep at you and tell you to break the pieces apart. In any case. you press the HELP button and it gives you little hints like.) The one we bought has an LCD screen that's maybe three inches square. and it sets itself up and takes off.sam. the attraction for me was that the menu-driven interface actually seemed simpler and more inviting than the ovens with timing buttons and 24 others marked "popcorn. you want everything else .com).) We bought a Sharp unit with the Interactive Display feature. and hit start. You pick one of those tasks. You do as you are told." "sandwich. Cook.to be on the full line voltage not affected by any power control scheme or timer. cover the edges. But. Just the the World needs is a smart microwave. how many pounds. Enter Quantity Press Start
Unit turns on and starts cooking. or CUT IN HALF.repairfaq.

But that popcorn button sure is a good selling point! :)
Microwave ovens for non-standard applications
Occasionally. :-( My first recommendation (though this is too weak a term) would to not do this. not reengineer the sound source .a microwave can develop hot spots . Use this to identify the safe limits front and back. We cut out squares (4"x 4") in the glass front and metal back of the oven to allow these strips to pass through the field. then special tests MUST be done to assure the safety of the users of the equipment. use a good conductive sheet metal such as copper or aluminum to reduce the size of the opening as close to the material as possible.) Our microwave has a button for popcorn.Notes on the Troubleshooting and Repair of Microwave Ovens
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have against a lot of mainstream HiFi equipment these days. The leakage detector or microwave field strength meter should come with information on acceptable
http://www. They consist of a ceramic (expanded alumina or something similar) insulating cylinder lined with a microwave susceptor . The smaller the opening. softening sticks of Dynamite is probably not to be recommended! (There actually is a reason for this .org/sam/micfaq. having said that I feel much better. The operators may have to remain further away or some additional shields may needed if these distances are not satisfactory. I can choose 5 minutes with two presses (QUICK.possibly a ferrite material. START). borrow an accurate microwave leakage detector. So on principal I choose 5 minutes on high and stop it at 1:45 (why not set for 3:15? because the one time I tried it the popcorn was burnt!). In general." Geez!!! You guys are out of your collective mind. if you insist.. etc. However. The manual says to monitor the popcorn anyway since it varies based on bag size. The wood stock will tend to reduce leakage while it is in place but the opening will leak like crazy when there is nothing in the hole. Next. See for example Microwave Melting of Metals. I am concerned about potential microwave leakage of a harmful nature. Sorry. these have to be taken on a case-by-case basis.htm
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.ca). I just want to listen to the music. Temperatures exceeding 1000 degrees C (yellow-white heat) are possible after a few minutes on high. A large appliance repair shop or electronics store may rent you one if you are persistent enough. the less will be the leakage. make sure there is always a load in the oven (a cup of water. The following is one such case in point: "My Dad and I are using a microwave oven to heat oak strips by passing them through the microwave field of a 1000W oven.repairfaq. Do your dynamite softening in a normal oven). 5) and popcorn with two presses (POPCORN. The sheet metal must be in electrical contact with the mesh in the door and the metal back. Obviously. people ask questions about the use of a microwave oven to do things other than heating food.) (From: Andrew Webber (webbers@magma.. Label these and don't go closer while the oven is in operation. As far as I can tell.heating is not as uniform as with normal ovens. for example) to keep the magnetron happy. Special kilns that will fit inside a microwave oven are apparently available to achieve really high temperatures. If any modifications are made to the oven that would compromise the integrity of the door seals or provide other places where microwave radiation could escape. Also. My second (and up to N where N is a very large number) recommendation would be not to do this. all it does is automatically set 5 minutes.

Thus. Don't set it next the opening as you hit START! This will prevent the possibility of damage to the expensive leakage tester (which could be costly) and exposure risk to you as well.) Field density and exposure time is a large factor. I think the story goes that this was how the heating affect was originally discovered. it doesn't make sense to take chances. One tends to remove one's hand when one senses heat. barring any reflective surface. The eye is particularly sensitive to this and it doesn't take much of an increase in temperature to denature the tissue of the central nervous system (i. Never let one go out of the shop unless the stirrer is turning. In addition the stirrer bearing will sometimes arc and may melt at the spots where it arcs.
http://www.check it out. This forces air over the stirrer when the cover is replaced. The number one precaution I've always held near and dear to me is to protect one's eyes.e. remove the grease shield and check the stirrer for burns that are causing it to stick.) You did the right thing to discourage people from breaching the integrity of a microwave oven. What is wrong with radiant heat??? (From Barry Collins (bcollins@mindspring. (I met one of the Japanese engineers who had unintentionally placed his head in a test oven that was working. there is no assurance that even this limit is safe. scramble your brain).htm
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. If this is ok or you correct it and stirrer still does not turn. If the stirrer does not turn. The Narda manual has multiple warning in it about this. I personally don't feel uncomfortable with what the person was doing.Notes on the Troubleshooting and Repair of Microwave Ovens
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power limits. but no lasting damage. because there are so many factors involved that one has to assume personal (or property) injury (or damage) may result from such actions. while the dangers may be overstated. If stirrer still does not turn. Just don't stand in front of the source. CAUTION (In addition to the loony nature of this entire project!): Since the leakage you encounter may be orders of magnitude greater than what is typical of even a misaligned microwave oven. but has one small modification which you will see when you compare the two. Power does fall off with the square of the distance and microwaves.) Everything depends on "Air Flow". are very directional by nature. start with the probe at a distance of a few feet and slowly move it closer while watching the meter or readout. you will always get a "Hot! spot" on the left bottom of the door. The human body does not have an adequate warning system since nerve endings sensitive to heat are somewhat sparse.
Short course on Amana
(From: Charles Godard (cgodard@iamerica. However. remove the cover and replace the foam gasket material. then replace the grease shield with a later model that looks almost the same as the original.. It is this that you might want to point out in your FAQ's. Next time it may be a cavity or magnetron overload that has opened due to the stirrer not turning. If your blower is running up to speed. It will soon be back unless all they do is heat coffee. This can happen much faster than with fleshy parts of the body and don't heal anywhere near the way a flesh injury does. The aqueous membranes of the eyes are perfect absorption material for stray microwaves.net).com). provided they had taken reasonable precautions (too numerous to list).repairfaq. It is something like 2 mW per square cm a foot or so from the oven .org/sam/micfaq. He reported warmth. aside from the resulting joke. The only known confirmed danger from microwave radiation is from internal heating effects.

Why Microwave-Safe Containers Get Destroyed
You probably have a cabinet full of so-called microwave-safe containers that look like they have been exposed to damage from a nuclear explosion. Keep in mind that there is a computer of sorts inside the microwave controlling it! However. interlocks. I doubt it is the microwave radiation itself doing anything to the material of the container directly and complaining to the oven manufacturer isn't likely to be very satisfying. :)
Back to Microwave Oven Repair FAQ Table of Contents.repairfaq. Why? It probably comes down to unequal heating of the contents or heating continuing long past the point where boiling takes place. This is entirely sufficient to deal with any problems in the microwave generator. especially the rear. But if the contents of the microwavesafe container are being heated. The problem would more likely be the magnetic field from the large transformer in the microwave oven causing interference on your monitor (wiggling. shimmering. etc. However. jiggling. I have gotten conflicting answers from the guy who rebuilt my computer and the guys at Radio Shack. a service manual is rarely required. I wish I had a hundred restaurant customers using them. The older Amana's power stays near 1500 watts forever.htm
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. protectors.
Service Information
Advanced troubleshooting
If the solutions to your problems have not been covered in this document. My feeling is that it should be fine. due to its effect on the electron beams in the CRT). and wiring." Did the kids at Radio Shack even understand the question??? :) Your request is certainly a bit unusual. Whether these are available depends on the manufacturer. What a waste!
Computer system near microwave oven?
"Can placing my microwave oven in close proximity to my computer and printer do any damage to either of them? The back of the oven would be right next to the printer and about 16 inches from the computer. you still have some options other than surrendering your microwave to the local service center or the dumpster. Retail customers are junking them because of $100 . No adjustments or alignment should even be required so detailed procedures for these are not needed. then some parts will get much hotter than others resulting in local melting and other damage. fuses. Unlike most other types of consumer electronic equipment. I would assume that putting a microwave-safe container in an oven with a cup of water in a separate container wouldn't result in any damage to the microwave-safe container. a service manual with schematics will prove essential. There should be no significant microwave leakage from the oven. when tackling electronic faults in the controller.org/sam/micfaq. some
http://www.Notes on the Troubleshooting and Repair of Microwave Ovens
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It's good work on a quality product. A sufficiently detailed schematic is nearly always pasted to the inside of the cover and includes all power components. you will need separate grounded electrical circuits for the microwave and computer equipment if you intend to ever use them at the same time. For legal reasons.to $125 repair bills.

83 or 683. Triac: $12. either. Power fuse: $. PA 17294-0850 ISBN 0-8306-6457-2 (hard).83 if your library is numbered that way) or from a technical bookstore. Overtemperature thermostat (thermal protector): $4. a division of McGraw Hill. Eagle Grove.50.
Suggested Reference
I know of at least one book dealing specifically with microwave oven repair. Touchpads and controller parts like the microprocessor chip are usually only available from the manufacturer of the oven.repairfaq.
Cost of repair parts
Assuming you have located one or more bad components. Interlock Switch: $2. they do not fail that often. 1991 Blue Ridge Summit. It should be possible to replace these with the $2 variety with wire leads). Voice phone: 1-800-5221264. Microwave Oven Repair. There is a good chance that your specific problem is covered. Prices are high . Magnetron: $30-100. Here is one place that seems to stock some: AMI Parts.. they won't be cheap . the question is whether an oven that is a few years old is worth fixing.50.a touchpad may cost $30 or more. Inc.org/sam/micfaq. Typical parts cost for generic replacements: HV diode: $2-5 (except for the bolt-on variety which can range up to $50. Sensors and other manufacturer specific parts will be expensive. They are concerned with litigation should an unqualified person be injured or killed. ISBN 0-8306-3457-6 (pbk. While the HV transformers are fairly standard. Parts suppliers like MCM Electronics can provide these components to fit the vast majority of microwave ovens. 2nd Edition Homer L.try the generic variety). 1. However. Davidson TAB Books. However.00 (unless original replacement in which case you will need to take out a mortgage . HV Capacitor: $10-20. MCM Electronics now lists at least one Goldstar model replacement.
http://www. Common generic replacements are $30-40. It is very complete and includes many actual repair case histories. IA.Notes on the Troubleshooting and Repair of Microwave Ovens
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manufacturers are reluctant to sell service information or replacement parts for microwave ovens.) This may be available at your public library (621. they are not readily available from the common replacement parts sources.expect to pay $50 or more!!! In addition.40.htm
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.

most of these have similar electrical ratings so a substitution is possible if you can make it fit physically. Many of these are interchangeable. use an exact replacement. In addition. it doesn't have to be from the original manufacturer .org/sam/micfaq. you can expect a repaired oven to behave just about like new. it will heat up very
http://www. while an exact match may be required.an exact replacement part is needed to maintain the specifications within acceptable limits with respect to line isolation. the answer is generally NO . 3. HV diode . a smaller one can be used for testing. a similar type may be used..it may be acceptable to use a $2. orientation of the cooling fins.similar (within 5%) and at least equal working voltage.a large number of microwave ovens use the same basic type but the mounting arrangement . Relays and triacs . Therefore. This would be particularly desirable where your oven has one of those chassis mount $50 dollar varieties .same temperature and maximum current rating. and to minimize fire hazards.holes vs. except for a slight decrease in power output as the oven is used over the years and the magnetron ages.75 generic replacement. studs. However. a few year old oven may not be worth fixing if the problem is a bad magnetron or touchpad. HV capacitor .substitutes will generally work as long as their specifications meet or exceed those of the original. Of course. Fuses . interlock switches. For microwave ovens such parts include the power fuses. You must be able to mount it securely and flush against the same surface as the old one.like a magnetron which did not fit the waveguide properly.htm
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. Thus the organs from that carcass may be able to provide renewed vitality to your ailing microwave.repairfaq. This will probably be a ceramic 1-1/4" x 1/4" 15 or 20 A 250 V fast blow type. Thermal protectors . radiation emission. However.most parts are interchangeable. 4. Here are some guidelines: 1. For the repair. For testing only. 2.exact same current rating and at least equal voltage rating.fore. Creative mounting may be required. 7. if the cooling fins end up being on the wrong side.must have the same terminal configuration and at least equal current rating. However.
Interchangeability of components
The question may arise: If I cannot obtain an exact replacement or if I have another microwave oven carcass gathering dust. the value should be matched fairly closely or else other parts may be overloaded. There. there is little to go bad or deteriorate. For safety related items. Thus. this is one situation where higher capacity (uF) is not better. Fortunately. can I substitute a part that is not a precise match? Sometimes. 6. a secure fit is very important as well for it to perform its safety function. and anything else that could potentially lead to microwave radiation leakage . differ. 5. The power output is related to capacitance. it is not possible to substitute something from your junkbox unless it is from a microwave oven. etc. Note that the working voltage rating of these capacitors is not consistent with the way capacitors in other electronic equipment are specified and is usually the RMS voltage of the AC input from the HV transformer. this is simply desired to confirm a diagnosis and avoid the risk of ordering an expensive replacement and/or having to wait until it arrives. Magnetrons . You can safely substitute a not exact match for testing purposes IF you can make it fit the waveguide securely without gaps.Notes on the Troubleshooting and Repair of Microwave Ovens
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With the prices of microwave ovens dropping almost as fast as PCs. Interlock switches .

Fans and motors . Mica waveguide cover . The touchpad is likely to be custom both electrically and physically as well unless you have a similar model microwave to cannibalize. Make sure the replacement has at least as high a current rating as the original.increase the capacitor's uF rating by 10 percent and the power and magnetron heat dissipation will go up by 2. Mounting should not be a problem but don't just leave it loose . .5% (assuming the relationship is linear right around the nominal value).must be 3 wire heavy duty grounded type. thermistors. I have not confirmed this.htm
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. the voltage rating must be at least equat to that of the original. Controller and touchpad .similar ratings and base.
http://www. diodes. however. It can be higher but never never lower or you will probably be replacing it again in the very near future. Cordsets . Speed isn't so critical for a turntable but for a magnetron cooling fan. the impedance of a 1 uF capacitor at 60 Hz is about 2. Light bulb . 13. Mechanical timers . 9.I estimate that it is about a 1:4 change . Thus the power difference is not a straight percent for percent change .small parts like resistors.depends on the particular model.org/sam/micfaq. inadequate air flow will result in overheating and shutdown or failure. Now for the uF rating: Unlike a conventional power supply filter capacitor. Turntable and mode mixer components . 15. capacitors. 8.compatible switching and mounting arrangement.cut to match. Temperature sensors. As an example. use them. Too small a capacitor and the doubler will not produce full output. A 1 kW magnetron running on just over 3 kV RMS is about 10 K ohms. etc. These are really really rough calculations.
Can I substitute a slightly different HV capacitor for a blown one?
It is not always possible or convenient to obtain an exact replacement high voltage capacitor.speed/power and direction must match and mounting must be possible. Observe the color code! 16. the capacitor in a microwave is in a voltage doubler and effectively in series with the load (magnetron). What will the effects be of using one that is a slightly different value? First.if they fit. 11. 14.and will not be suitable as a permanent replacement. 10. Common shaded pole type motors may be interchangeable with other appliances or if a mounting arrangement can be cobbled together. Forget about the controller ICs or display. its value **does** have an impact on output power.50% of the input power goes to heat .as well as heat dissipation in the magnetron.you could end up with a disaster. A larger capacitor will slightly increase the output power . Therefore.5 K ohms. HV transformer . and so forth can often be substituted. The cap is in effect in series with the magnetron.Notes on the Troubleshooting and Repair of Microwave Ovens
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quickly . 12.repairfaq.same (within 5%) voltage and at least equal current rating.

Another alternative is to determine who actually made your oven.. but an inspection inside may reveal the actual manufacturer. This is obvious with name brands like Panasonic and Sharp. For example.unless they are convinced you are a certified repair technician. Also. However. HV capacitors and diodes. However. Then. I had to give up on a Sharp microwave/convection oven that was 15 years old because specialized replacement parts were no longer available from Sharp.
Obtaining replacement parts for microwave ovens
For general electronic components like resistors and capacitors. Your local appliance distributor or repair parts outlet may be able to obtain an exact replacement or something that is an ecceptable substitute. However. interlock switches.you have to convince them that you have at least the intelligence of an average carrot and possibly sign a 100+ page document written by too many lawyers.S. places like Digikey. Allied. Many companies will be happy to sell service parts but availability may be a problem on older ovens. Their prices are inflated as well. The following suppliers have web sites with on-line catalogs and list a very extensive selection of microwave oven parts. and standard door switches. needed for microwave oven repair.com/ Their web site includes a very extensive selection of microwave oven parts. complaints/attempts to return parts when a repair doesn't work.Notes on the Troubleshooting and Repair of Microwave Ovens
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Therefore. the cost will be higher than for generic parts from the places listed below if they carry what you need. go direct to the horse's mouth. certain companies (like Sears) may set their own rules . In the U. and the small quantities involved. most electronics distributors will have a sufficient variety at reasonable cost.htm
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. Note: I have heard that in other parts of the world. turntables. However.allapplianceparts. nearly 50 different magnetrons are listed along with little photos of each!
http://www. there may be restrictions on who can actually purchase microwave oven parts other than things like light bulbs. thermal protectors. :)
Sources for replacement microwave oven parts
See the document: Major Service Parts Suppliers for some companies that I have used in the past and others that have been recommended. I suppose this may be due to several factors including the potential liability issues. There is a chance that they may not want to sell to the general public. etc. Global/MPI/All Appliance Parts Phone: 1-800-325-8488 Web: http://www. Sears doesn't manufacture their own appliances. some places like Sears.. They may include microwave oven parts in their catalog but don't specialize in them.org/sam/micfaq. may refuse to sell you anything microwave oven related due to safety concerns . whatever that might mean. Even Radio Shack can be considered in a pinch. it is definitely worth checking as the public web sites implie a desire to deal with the entire Internet community. I would say that using a capacitor with up to a 10-15% difference (either way) in uF rating is probably acceptable but a closer match is better.repairfaq. Going direct to the manufacturer is a possibility but expect to pay more than might be charged for generic replacement parts by an independent company. and Newark do not have the specialized parts like magnetrons.